diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 384d3e2..f8a85a4 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -1,6 +1,6 @@
 # Pre-v1 Public API Cleanup Implementation Plan
 
-Status: in progress; Stages 1–6 are complete and Stages 7–11 are pending.
+Status: in progress; Stages 1–7 are complete and Stages 8–11 are pending.
 Baseline: `07e4ae5d9236935389e81b207d4a8f00543fafce`.
 Audience: a Luna coding agent executing the stages sequentially.
 
@@ -298,7 +298,7 @@ is unchanged.
 
 ## Stage 7 — Use Main Consistently In Provenance
 
-Status: pending. Implements N2.
+Status: complete. Implements N2.
 
 1. Replace `LayerBase = "base"` with `LayerMain = "main"` in root/engine
    records. Migrate producer references across acquisition, composition,
diff --git a/examples/consumers/destinations_test.go b/examples/consumers/destinations_test.go
index ff76eec..2086a23 100644
--- a/examples/consumers/destinations_test.go
+++ b/examples/consumers/destinations_test.go
@@ -60,7 +60,7 @@ func TestCollectionBundleAssemblesMountedPeersAndResources(t *testing.T) {
 	}
 
 	versionInfo, ok := result.Metadata().Lookup(configkit.Keys("version"))
-	if !ok || !versionInfo.Explicit || versionInfo.Origin.Layer != configkit.LayerBase || len(versionInfo.Origin.Locations) != 1 {
+	if !ok || !versionInfo.Explicit || versionInfo.Origin.Layer != configkit.LayerMain || len(versionInfo.Origin.Locations) != 1 {
 		t.Fatalf("main-owned version metadata = %#v, %v", versionInfo, ok)
 	}
 	collectionVersionPath := configkit.Keys("collections", "weatherreporter", "version")
diff --git a/examples/consumers/layered_test.go b/examples/consumers/layered_test.go
index 4802e31..4c571fb 100644
--- a/examples/consumers/layered_test.go
+++ b/examples/consumers/layered_test.go
@@ -119,7 +119,7 @@ func TestCatalogRetainsPresenceAndSourceContext(t *testing.T) {
 	}
 	for _, key := range []string{"enabled", "workers"} {
 		info, found := result.Metadata().Lookup(configkit.Keys(key))
-		if !found || !info.Explicit || info.Origin.Layer != configkit.LayerBase {
+		if !found || !info.Explicit || info.Origin.Layer != configkit.LayerMain {
 			t.Fatalf("%s presence = %#v, %v", key, info, found)
 		}
 	}
diff --git a/facade_destination_test.go b/facade_destination_test.go
index dcfe9b5..ee1b88b 100644
--- a/facade_destination_test.go
+++ b/facade_destination_test.go
@@ -405,7 +405,7 @@ func TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff(t *testing.T) {
 		if err != nil || string(data) != "source-relative summary" {
 			t.Fatalf("completed relative resource = %q, %v", data, err)
 		}
-		if info, ok := result.Metadata().Lookup(titlePath); !ok || info.Origin.Layer != configkit.LayerBase {
+		if info, ok := result.Metadata().Lookup(titlePath); !ok || info.Origin.Layer != configkit.LayerMain {
 			t.Fatalf("mounted title metadata = %#v, %v", info, ok)
 		}
 		if history := result.Metadata().History(titlePath); len(history) < 2 {
diff --git a/facade_discovery_test.go b/facade_discovery_test.go
index dd1f81c..880d6e6 100644
--- a/facade_discovery_test.go
+++ b/facade_discovery_test.go
@@ -142,7 +142,7 @@ func TestFacadeSearchWorksForLayeredMainAndUnderlay(t *testing.T) {
 		t.Fatalf("layered search = %#v, %v", result, err)
 	}
 	sources := result.Sources()
-	if len(sources) != 2 || sources[0].Path != canonicalFacadePath(t, mainPath) || sources[0].Layer != LayerBase ||
+	if len(sources) != 2 || sources[0].Path != canonicalFacadePath(t, mainPath) || sources[0].Layer != LayerMain ||
 		sources[1].Path != canonicalFacadePath(t, underlayPath) || sources[1].Layer != LayerUnderlay {
 		t.Fatalf("layered search sources = %#v", sources)
 	}
diff --git a/facade_equivalence_test.go b/facade_equivalence_test.go
index 1261dff..42a596a 100644
--- a/facade_equivalence_test.go
+++ b/facade_equivalence_test.go
@@ -69,7 +69,7 @@ func TestEquivalentSourceFormsPreserveInputSemantics(t *testing.T) {
 			}
 			for _, field := range []string{"version", "name", "enabled", "count", "items", "optional"} {
 				info, ok := result.Metadata().Lookup(configkit.Keys(field))
-				if !ok || !info.Explicit || info.Origin.Layer != configkit.LayerBase || info.Null != (field == "optional") {
+				if !ok || !info.Explicit || info.Origin.Layer != configkit.LayerMain || info.Null != (field == "optional") {
 					t.Errorf("%s input info = %#v, %v", field, info, ok)
 				}
 			}
diff --git a/facade_historical_paths_test.go b/facade_historical_paths_test.go
index 348f31b..d654cc2 100644
--- a/facade_historical_paths_test.go
+++ b/facade_historical_paths_test.go
@@ -85,16 +85,16 @@ func assertPublicShapeHistory(t *testing.T, metadata configkit.Metadata, operati
 	}
 	var keyDefinition, keyReplacement, indexReplacement bool
 	for _, change := range keyHistory {
-		keyDefinition = keyDefinition || change.Origin.Layer == configkit.LayerBase
+		keyDefinition = keyDefinition || change.Origin.Layer == configkit.LayerMain
 		keyReplacement = keyReplacement || change.Origin.Layer == operationLayer && change.Path.String() == "/shape"
 	}
 	for _, change := range indexHistory {
-		if change.Origin.Layer == configkit.LayerBase {
+		if change.Origin.Layer == configkit.LayerMain {
 			t.Fatalf("introduced index inherited mapping-key history: %#v", indexHistory)
 		}
 		indexReplacement = indexReplacement || change.Origin.Layer == operationLayer && change.Path.String() == "/shape"
 		for _, previous := range change.Previous {
-			if previous.Layer == configkit.LayerBase {
+			if previous.Layer == configkit.LayerMain {
 				t.Fatalf("introduced index inherited mapping-key previous origin: %#v", change)
 			}
 		}
diff --git a/facade_import_lifecycle_test.go b/facade_import_lifecycle_test.go
index a238134..ea966c1 100644
--- a/facade_import_lifecycle_test.go
+++ b/facade_import_lifecycle_test.go
@@ -67,6 +67,14 @@ entries:
 	}
 
 	sources := result.Sources()
+	if len(sources) < 3 || sources[0].Layer != LayerMain || string(sources[0].Layer) != "main" {
+		t.Fatalf("main source attribution = %#v", sources)
+	}
+	for _, source := range sources[1:] {
+		if source.ImportedFrom != nil && (source.Layer != LayerMain || string(source.Layer) != "main") {
+			t.Errorf("imported main source attribution = %#v", source)
+		}
+	}
 	filesByID := make(map[string]string, len(sources))
 	for _, source := range sources {
 		filesByID[source.ID] = source.Path
diff --git a/facade_layered_test.go b/facade_layered_test.go
index b3b3bd8..6dbb21a 100644
--- a/facade_layered_test.go
+++ b/facade_layered_test.go
@@ -80,7 +80,7 @@ func TestFacadeLayeredMixedSourcesAndInspection(t *testing.T) {
 	if len(sources) != 8 {
 		t.Fatalf("sources = %#v", sources)
 	}
-	wantLayers := []configkit.Layer{configkit.LayerBase, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerOverlay, configkit.LayerCLI}
+	wantLayers := []configkit.Layer{configkit.LayerMain, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerOverlay, configkit.LayerCLI}
 	wantIndexes := []int{0, 1, 1, 1, 1, 2, 0, 0}
 	wantKinds := []configkit.SourceKind{configkit.SourceKindFile, configkit.SourceKindFilesystem, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindCLI}
 	for index, source := range sources {
diff --git a/facade_mapping_test.go b/facade_mapping_test.go
index ce1cd83..667a8a4 100644
--- a/facade_mapping_test.go
+++ b/facade_mapping_test.go
@@ -36,7 +36,7 @@ func TestFacadeMappingSnapshotsAndMatchesYAML(t *testing.T) {
 		if !reflect.DeepEqual(result.Config, want) {
 			t.Fatalf("mapping config = %#v, want %#v", result.Config, want)
 		}
-		if sources := result.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Kind != configkit.SourceKindMapping || sources[0].Layer != configkit.LayerBase || sources[0].Path != "" {
+		if sources := result.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Kind != configkit.SourceKindMapping || sources[0].Layer != configkit.LayerMain || sources[0].Path != "" {
 			t.Fatalf("mapping sources = %#v", sources)
 		}
 		info, ok := result.Metadata().Lookup(configkit.Keys("name"))
@@ -89,7 +89,7 @@ func TestFacadeMappingImportsUseExplicitContext(t *testing.T) {
 	if err != nil || result.Config.Name != "imported" || !result.Config.Enabled {
 		t.Fatalf("mapping import = %#v, %v", result, err)
 	}
-	if sources := result.Sources(); len(sources) != 2 || sources[0].Kind != configkit.SourceKindMapping || sources[1].Kind != configkit.SourceKindFilesystem || sources[1].Layer != configkit.LayerBase {
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Kind != configkit.SourceKindMapping || sources[1].Kind != configkit.SourceKindFilesystem || sources[1].Layer != configkit.LayerMain {
 		t.Fatalf("mapping import sources = %#v", sources)
 	}
 	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{Imports: true}, configkit.CodeOptionsInvalid)
diff --git a/facade_sequence_test.go b/facade_sequence_test.go
index 6c175ec..290fdfe 100644
--- a/facade_sequence_test.go
+++ b/facade_sequence_test.go
@@ -125,7 +125,7 @@ func TestFacadeRegisteredReplacementExplainsRemovedDescendants(t *testing.T) {
 				t.Fatalf("removal not explained: %#v", explanation)
 			}
 			last := explanation.History[len(explanation.History)-1]
-			if !last.Before.Present || last.After.Present || last.Origin.Layer != configkit.LayerBase || len(last.Origin.Locations) != 1 || last.Origin.Locations[0].SourceID != result.Sources()[0].ID {
+			if !last.Before.Present || last.After.Present || last.Origin.Layer != configkit.LayerMain || len(last.Origin.Locations) != 1 || last.Origin.Locations[0].SourceID != result.Sources()[0].ID {
 				t.Fatalf("wrong removal transition/origin: %#v", last)
 			}
 		})
@@ -612,7 +612,7 @@ entries:
 					t.Error("resolver treated an omitted field as explicit")
 				}
 				info, found := resolution.EntryInfo(configkit.Keys("entries").Index(0).Key("count"))
-				if !found || info.Origin.Layer != configkit.LayerBase {
+				if !found || info.Origin.Layer != configkit.LayerMain {
 					t.Errorf("resolver current-record input info = %#v, %v", info, found)
 				}
 				for _, id := range []string{"source-3", "source-4"} {
diff --git a/facade_sequence_version_test.go b/facade_sequence_version_test.go
index 381837b..d30b2f6 100644
--- a/facade_sequence_version_test.go
+++ b/facade_sequence_version_test.go
@@ -64,7 +64,7 @@ func TestMainVersionDecodingFollowsSequenceMovement(t *testing.T) {
 						t.Fatal("unrelated underlay payload was treated as the main declaration")
 					}
 					for _, source := range loaded.Sources() {
-						if source.Layer == configkit.LayerBase && source.VersionEvidence.Path.String() != path.String() {
+						if source.Layer == configkit.LayerMain && source.VersionEvidence.Path.String() != path.String() {
 							t.Fatal("effective movement changed source-local version evidence")
 						}
 					}
diff --git a/internal/engine/compose.go b/internal/engine/compose.go
index 8c23bfd..71bd600 100644
--- a/internal/engine/compose.go
+++ b/internal/engine/compose.go
@@ -223,7 +223,7 @@ func composeDocumentsWithCollectionPolicy(
 		rangesBySource[assigned.sourceIndex] = append(rangesBySource[assigned.sourceIndex], assigned)
 	}
 	for sourceIndex, input := range inputs {
-		application := historyApplication{layer: LayerBase, value: input}
+		application := historyApplication{layer: LayerMain, value: input}
 		for _, assigned := range rangesBySource[sourceIndex] {
 			pointer := assigned.path.String()
 			mode := sequenceHistoryAppend
@@ -255,7 +255,7 @@ func composeDocumentsWithCollectionPolicy(
 func completeCompositionOrigins(node *document.Node, path Path, origins map[string]Origin) {
 	pointer := path.String()
 	if _, present := origins[pointer]; !present {
-		origins[pointer] = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+		origins[pointer] = Origin{Layer: LayerMain, Locations: []Location{publicLocation(node.Location)}}
 	}
 	switch node.Kind {
 	case document.Mapping:
@@ -513,7 +513,7 @@ func projectedCompositionOrigins(inputs []*document.Node, ranges []sequenceRange
 		walkHistoryApplication(application, func(_, effective Path, node *document.Node) {
 			pointer := effective.String()
 			origin := origins[pointer]
-			origin.Layer = LayerBase
+			origin.Layer = LayerMain
 			origin.Locations = append(origin.Locations, publicLocation(node.Location))
 			origins[pointer] = origin
 		})
diff --git a/internal/engine/compose_test.go b/internal/engine/compose_test.go
index 053bfe8..5b1a5bc 100644
--- a/internal/engine/compose_test.go
+++ b/internal/engine/compose_test.go
@@ -92,7 +92,7 @@ values: [one, two]
 		{SourceID: "source-1", Line: 2, Column: 3},
 		{SourceID: "source-2", Line: 2, Column: 3},
 	}
-	if !reflect.DeepEqual(serviceOrigin, Origin{Layer: LayerBase, Locations: wantLocations}) {
+	if !reflect.DeepEqual(serviceOrigin, Origin{Layer: LayerMain, Locations: wantLocations}) {
 		t.Fatalf("services origin = %#v, want composite origin", serviceOrigin)
 	}
 	wantClaimPaths := []string{"", "/services", "/services/api", "/services/api/enabled", "/empty", "", "/services", "/services/worker", "/services/worker/enabled", "/values"}
diff --git a/internal/engine/decode.go b/internal/engine/decode.go
index 1fed3c5..b6576a4 100644
--- a/internal/engine/decode.go
+++ b/internal/engine/decode.go
@@ -448,7 +448,7 @@ func collectPresence(node *document.Node, path Path, presence map[string]inputPr
 
 func collectOrigins(node *document.Node, path Path, presence map[string]inputPresence, origins map[string]Origin) {
 	if input, ok := presence[path.String()]; ok {
-		origins[path.String()] = Origin{Layer: LayerBase, Locations: []Location{{SourceID: input.location.SourceID, Line: input.location.Line, Column: input.location.Column}}}
+		origins[path.String()] = Origin{Layer: LayerMain, Locations: []Location{{SourceID: input.location.SourceID, Line: input.location.Line, Column: input.location.Column}}}
 	} else {
 		origins[path.String()] = Origin{Layer: LayerDefault}
 	}
diff --git a/internal/engine/decode_test.go b/internal/engine/decode_test.go
index 91acf85..f717a8c 100644
--- a/internal/engine/decode_test.go
+++ b/internal/engine/decode_test.go
@@ -141,7 +141,7 @@ custom: accepted
 	if _, ok := decoded.presence[Keys("nested", "a").String()]; ok {
 		t.Fatal("parent mapping made omitted child explicit")
 	}
-	if decoded.origins[Keys("nested", "a").String()].Layer != LayerDefault || decoded.origins[Keys("nested", "b").String()].Layer != LayerBase {
+	if decoded.origins[Keys("nested", "a").String()].Layer != LayerDefault || decoded.origins[Keys("nested", "b").String()].Layer != LayerMain {
 		t.Fatalf("origins = %#v", decoded.origins)
 	}
 	if decoded.origins[Keys("nested", "b").String()].Locations[0].Line == 0 {
diff --git a/internal/engine/discovery.go b/internal/engine/discovery.go
index c6f02bb..10f92e6 100644
--- a/internal/engine/discovery.go
+++ b/internal/engine/discovery.go
@@ -481,7 +481,7 @@ func resolveSearchInput(source Source, sourceID string, helpers discoveryDirecto
 	if len(diagnostics) == 0 || selected.kind != sourceFile {
 		return selected, nil, diagnostics
 	}
-	info := &SourceInfo{ID: sourceID, Label: selected.label, Path: selected.path, Layer: LayerBase, Kind: SourceKindFile}
+	info := &SourceInfo{ID: sourceID, Label: selected.label, Path: selected.path, Layer: LayerMain, Kind: SourceKindFile}
 	for index := range diagnostics {
 		diagnostics[index].Primary = &Location{SourceID: sourceID}
 	}
diff --git a/internal/engine/discovery_load_test.go b/internal/engine/discovery_load_test.go
index 56a5f85..876b2d6 100644
--- a/internal/engine/discovery_load_test.go
+++ b/internal/engine/discovery_load_test.go
@@ -77,7 +77,7 @@ settings: {underlay: value}
 		Keys("settings", "main"), Keys("settings", "underlay"), Keys("settings", "imported"))
 
 	sources := searched.Inspection.Sources()
-	if len(sources) != 3 || sources[0].ID != "source-1" || sources[0].Layer != LayerBase || sources[1].ID != "source-2" || sources[1].Layer != LayerUnderlay || sources[2].Path != canonicalTestPath(t, importPath) || sources[2].UnderlayIndex != 1 {
+	if len(sources) != 3 || sources[0].ID != "source-1" || sources[0].Layer != LayerMain || sources[1].ID != "source-2" || sources[1].Layer != LayerUnderlay || sources[2].Path != canonicalTestPath(t, importPath) || sources[2].UnderlayIndex != 1 {
 		t.Fatalf("layered search source evidence = %#v", sources)
 	}
 }
diff --git a/internal/engine/explain_render.go b/internal/engine/explain_render.go
index 4b73cb6..43d3bce 100644
--- a/internal/engine/explain_render.go
+++ b/internal/engine/explain_render.go
@@ -44,7 +44,7 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 			rendered.WriteString("input to canonical typed\n")
 		} else if change.Origin.Operation == ChangeInheritance {
 			rendered.WriteString("structural inheritance\n")
-		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerUnderlay || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
+		} else if change.Origin.Layer == LayerMain || change.Origin.Layer == LayerUnderlay || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
 			rendered.WriteString("policy-validated input\n")
 		} else {
 			rendered.WriteString("canonical typed\n")
diff --git a/internal/engine/explain_test.go b/internal/engine/explain_test.go
index b1c9288..a3e7940 100644
--- a/internal/engine/explain_test.go
+++ b/internal/engine/explain_test.go
@@ -137,7 +137,7 @@ dynamic: selected
 	if got := explanationValues(concurrency); !reflect.DeepEqual(got, []string{"4\n", "8\n", "1\n", "2\n", "3\n"}) {
 		t.Fatalf("concurrency history = %#v", got)
 	}
-	if got := historyValueLayers(concurrency); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerOverlay, LayerCLI, LayerResolution}) {
+	if got := historyValueLayers(concurrency); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerMain, LayerOverlay, LayerCLI, LayerResolution}) {
 		t.Fatalf("concurrency layers = %#v", got)
 	}
 	if concurrency.History[4].Before.YAML != "2\n" || concurrency.History[4].Reasons[0] != "normalized\nconcurrency" ||
@@ -265,7 +265,7 @@ dynamic: selected
 	var invalidInput bytes.Buffer
 	if err := result.WriteExplanation(&invalidInput, Keys("limit")); err != nil ||
 		!strings.Contains(invalidInput.String(), "policy-validated input") ||
-		strings.Contains(invalidInput.String(), "wrong-before-overlay\n    source: layer=\"base\"\n    before (canonical typed)") {
+		strings.Contains(invalidInput.String(), "wrong-before-overlay\n    source: layer=\"main\"\n    before (canonical typed)") {
 		t.Fatalf("historical representation rendering = %v\n%s", err, invalidInput.String())
 	}
 }
@@ -312,7 +312,7 @@ func TestHistoryNodeBudgetCountsSharedTreesOnce(t *testing.T) {
 	builder.model.baseline = root
 	builder.model.final = root
 	builder.model.sourcePayloads = []historySourcePayload{{
-		kind: historySourceApplicationPayload, application: historyApplication{layer: LayerBase, value: root},
+		kind: historySourceApplicationPayload, application: historyApplication{layer: LayerMain, value: root},
 	}}
 	model := builder.freeze()
 	inspection := &Inspection{history: model, metadata: Metadata{historyModel: model}, final: root}
@@ -325,7 +325,7 @@ func TestHistoryNodeBudgetCountsSharedTreesOnce(t *testing.T) {
 	builder.model.baseline = root
 	builder.model.final = root
 	builder.model.sourcePayloads = []historySourcePayload{{
-		kind: historySourceApplicationPayload, application: historyApplication{layer: LayerBase, value: other},
+		kind: historySourceApplicationPayload, application: historyApplication{layer: LayerMain, value: other},
 	}}
 	model = builder.freeze()
 	inspection.history = model
@@ -445,7 +445,7 @@ func TestIntegerVersionInspectionRetainsInputAndReconcilesTypedValue(t *testing.
 	}
 
 	info, ok := first.Metadata().Lookup(Keys("version"))
-	if !ok || !info.Explicit || info.Null || info.Origin.Layer != LayerBase || len(info.Origin.Locations) != 1 ||
+	if !ok || !info.Explicit || info.Null || info.Origin.Layer != LayerMain || len(info.Origin.Locations) != 1 ||
 		info.Origin.Locations[0].SourceID != "source-1" || info.Origin.Locations[0].Line != 1 {
 		t.Fatalf("version metadata = %#v, %v", info, ok)
 	}
@@ -455,7 +455,7 @@ func TestIntegerVersionInspectionRetainsInputAndReconcilesTypedValue(t *testing.
 	}
 	foundInput, foundDecoding := false, false
 	for _, change := range explanation.History {
-		if change.After.YAML == "4\n" && change.Origin.Layer == LayerBase {
+		if change.After.YAML == "4\n" && change.Origin.Layer == LayerMain {
 			foundInput = true
 		}
 		if change.Before.YAML == "4\n" && change.After.YAML == "'4'\n" && change.Origin.Operation == ChangeDecoding {
diff --git a/internal/engine/history_cli_test.go b/internal/engine/history_cli_test.go
index f405bfa..40da32f 100644
--- a/internal/engine/history_cli_test.go
+++ b/internal/engine/history_cli_test.go
@@ -38,7 +38,7 @@ func TestCLIOverrideRecordsTrackConcreteShapeChanges(t *testing.T) {
 			setOverrideTestDocumentSource(incomingRoot, "cli-source")
 			incoming, _ := documentAtPath(incomingRoot, Keys("target"))
 			origins := make(map[string]Origin)
-			collectOverlayOrigins(origins, before, Path{}, LayerBase)
+			collectOverlayOrigins(origins, before, Path{}, LayerMain)
 			records := overrideMetadataRecords(decodedConfiguration{final: before, origins: origins}, []preparedOverride{{
 				path: Keys("target"), value: incoming, source: SourceInfo{ID: "cli-source"},
 			}})
@@ -83,7 +83,7 @@ func TestCLIHistoryEvidenceKeepsCheckpointsAndConditionalReconciliation(t *testi
 	structural := parseDocumentForDecode(t, "count: 7\nsettings: {a: 1, b: 2}\nitems: {old: value}\n")
 	preCLI := parseDocumentForDecode(t, "count: '7'\nsettings: {a: 1, b: 2}\nitems: {old: value}\ndefaulted: 9\n")
 	preOrigins := map[string]Origin{
-		Keys("count").String():     {Layer: LayerBase, Locations: []Location{{SourceID: "input"}}},
+		Keys("count").String():     {Layer: LayerMain, Locations: []Location{{SourceID: "input"}}},
 		Keys("defaulted").String(): {Layer: LayerDefault},
 	}
 	countRoot := parseDocumentForDecode(t, "count: 7\n")
diff --git a/internal/engine/history_inheritance_test.go b/internal/engine/history_inheritance_test.go
index 87edac8..e8da759 100644
--- a/internal/engine/history_inheritance_test.go
+++ b/internal/engine/history_inheritance_test.go
@@ -14,9 +14,9 @@ func TestInheritanceEvidenceUsesOrderedPayloadAndConcreteTransferShapes(t *testi
 	effective := parseDocumentForDecode(t, "id: child\nbranch: replaced\n")
 	root := parseDocumentForDecode(t, "parent: {unused: true}\nchild: {id: child, extends: parent, branch: {local: stale}}\n")
 	parentOrigins := map[string]Origin{
-		Keys("parent", "branch").String():                 {Layer: LayerBase, Locations: []Location{{SourceID: "parent"}}},
-		Keys("parent", "branch", "deep").String():         {Layer: LayerBase, Locations: []Location{{SourceID: "parent"}}},
-		Keys("parent", "branch", "deep", "leaf").String(): {Layer: LayerBase, Locations: []Location{{SourceID: "parent"}}},
+		Keys("parent", "branch").String():                 {Layer: LayerMain, Locations: []Location{{SourceID: "parent"}}},
+		Keys("parent", "branch", "deep").String():         {Layer: LayerMain, Locations: []Location{{SourceID: "parent"}}},
+		Keys("parent", "branch", "deep", "leaf").String(): {Layer: LayerMain, Locations: []Location{{SourceID: "parent"}}},
 	}
 	effectiveOrigins := map[string]Origin{
 		Keys("child", "branch").String(): {Layer: LayerOverlay, Locations: []Location{{SourceID: "child"}}},
@@ -76,7 +76,7 @@ func TestInheritanceModelSharesProjectionAttribution(t *testing.T) {
 
 	from, to, reference := Keys("parents", "base"), Keys("children", "item"), Keys("parents", "base")
 	path := to.Key("nested").Key("value")
-	parent := Origin{Layer: LayerBase, Locations: []Location{{SourceID: "base"}}}
+	parent := Origin{Layer: LayerMain, Locations: []Location{{SourceID: "base"}}}
 	child := Origin{Layer: LayerOverlay, Locations: []Location{{SourceID: "child"}}}
 	parentDocument := parseDocumentForDecode(t, "nested: {value: parent}\n")
 	effectiveDocument := parseDocumentForDecode(t, "nested: {value: child}\n")
@@ -121,7 +121,7 @@ func TestInheritanceEvidenceSeparatesCollidingKeyAndIndexShapes(t *testing.T) {
 	childPath := to.Key("items").Key("0").Key("value")
 	operation := inheritanceOperation{
 		transfer: &transfer, parent: parent, effective: effective,
-		parentOrigins:    map[string]Origin{parentPath.String(): {Layer: LayerBase, Locations: []Location{{SourceID: "parent"}}}},
+		parentOrigins:    map[string]Origin{parentPath.String(): {Layer: LayerMain, Locations: []Location{{SourceID: "parent"}}}},
 		effectiveOrigins: map[string]Origin{childPath.String(): {Layer: LayerOverlay, Locations: []Location{{SourceID: "child"}}}},
 		childOrigins:     map[string]Origin{childPath.String(): {Layer: LayerOverlay, Locations: []Location{{SourceID: "child"}}}},
 	}
diff --git a/internal/engine/history_select_test.go b/internal/engine/history_select_test.go
index 00245c7..37a6d3d 100644
--- a/internal/engine/history_select_test.go
+++ b/internal/engine/history_select_test.go
@@ -58,7 +58,7 @@ func TestHistorySelectionSeparatesNumericShapesAndGroupsParsedSides(t *testing.T
 	builder.observeDocument(mapping, historyPhaseBaseline)
 	builder.observeDocument(sequence, historyPhaseResolution)
 
-	baseOrigin := Origin{Layer: LayerBase}
+	baseOrigin := Origin{Layer: LayerMain}
 	baseAttribution := builder.addAttribution(baseOrigin, nil, nil)
 	keyPath := Keys("shape", "0")
 	indexPath := Keys("shape").Index(0)
@@ -125,7 +125,7 @@ func TestHistorySelectionSeparatesNumericShapesAndGroupsParsedSides(t *testing.T
 	if len(keySelections) != 2 || keySelections[0].role != historyRoleDefinition || keySelections[1].before != true || keySelections[1].after {
 		t.Fatalf("mapping-key selections = %#v", keySelections)
 	}
-	if len(keySelections[1].previous) != 1 || keySelections[1].previous[0].Layer != LayerBase {
+	if len(keySelections[1].previous) != 1 || keySelections[1].previous[0].Layer != LayerMain {
 		t.Fatalf("mapping-key previous origins = %#v", keySelections[1].previous)
 	}
 
@@ -163,7 +163,7 @@ func TestHistorySelectionMatchesDeepSidesOfAncestorShapeChange(t *testing.T) {
 	builder := newHistoryBuilder()
 	builder.observeDocument(parseDocumentForDecode(t, "shape: {'0': {leaf: old}}\n"), historyPhaseSources)
 	builder.observeDocument(parseDocumentForDecode(t, "shape: [{leaf: new}]\n"), historyPhaseResolution)
-	base := builder.addAttribution(Origin{Layer: LayerBase}, nil, nil)
+	base := builder.addAttribution(Origin{Layer: LayerMain}, nil, nil)
 	oldPath := Keys("shape", "0", "leaf")
 	newPath := Keys("shape").Index(0).Key("leaf")
 	definition := builder.nextPayload(historyPhaseSources)
@@ -187,7 +187,7 @@ func TestHistorySelectionMatchesDeepSidesOfAncestorShapeChange(t *testing.T) {
 		t.Fatalf("new descendant selections = %#v", newSelections)
 	}
 	oldSelections := model.selectHistory(oldPath)
-	if len(oldSelections) != 2 || !oldSelections[1].before || oldSelections[1].after || len(oldSelections[1].previous) != 1 || oldSelections[1].previous[0].Layer != LayerBase {
+	if len(oldSelections) != 2 || !oldSelections[1].before || oldSelections[1].after || len(oldSelections[1].previous) != 1 || oldSelections[1].previous[0].Layer != LayerMain {
 		t.Fatalf("removed descendant selections = %#v", oldSelections)
 	}
 	if selections := model.selectHistory(Keys("shape").Index(0).Key("missing")); len(selections) != 0 {
@@ -207,7 +207,7 @@ func TestHistoryModelFinalizesLocationAndSensitivityWithoutRetainingInventoryTre
 	builder := newHistoryBuilder()
 	builder.observeDocument(root, historyPhaseSources)
 	definitionAttribution := builder.addAttribution(Origin{
-		Layer: LayerBase, Locations: []Location{{SourceID: "base.yml", Line: 4, Column: 2}},
+		Layer: LayerMain, Locations: []Location{{SourceID: "base.yml", Line: 4, Column: 2}},
 	}, nil, nil)
 	definition := builder.nextPayload(historyPhaseSources)
 	builder.addObservation(historyObservation{
diff --git a/internal/engine/history_sequence_test.go b/internal/engine/history_sequence_test.go
index 7dd09ef..69fb7c7 100644
--- a/internal/engine/history_sequence_test.go
+++ b/internal/engine/history_sequence_test.go
@@ -107,7 +107,7 @@ func TestTierSequenceHistoryReplaysSelectedOperation(t *testing.T) {
 				t.Fatal(diagnostics)
 			}
 			assembly := newAssemblyAccumulator(lower)
-			if diagnostics := assembly.applyOverlayWithCollectionPolicy(higher.root, higher.origins, higher.collectionPolicies, higher.applications, LayerBase, policy, declarations); len(diagnostics) != 0 {
+			if diagnostics := assembly.applyOverlayWithCollectionPolicy(higher.root, higher.origins, higher.collectionPolicies, higher.applications, LayerMain, policy, declarations); len(diagnostics) != 0 {
 				t.Fatal(diagnostics)
 			}
 			if len(assembly.current.applications) != 3 {
@@ -176,7 +176,7 @@ func TestSequenceHistoryPreservesTierReplacementAndEmptyLists(t *testing.T) {
 	if diagnostics := assembly.applyOverlay(second.root, second.origins, second.applications, LayerUnderlay); len(diagnostics) != 0 {
 		t.Fatalf("second tier handoff diagnostics = %#v", diagnostics)
 	}
-	if diagnostics := assembly.applyOverlay(main.root, main.origins, main.applications, LayerBase); len(diagnostics) != 0 {
+	if diagnostics := assembly.applyOverlay(main.root, main.origins, main.applications, LayerMain); len(diagnostics) != 0 {
 		t.Fatalf("main handoff diagnostics = %#v", diagnostics)
 	}
 	if len(assembly.current.applications) != 4 {
@@ -201,7 +201,7 @@ func TestSequenceHistoryPreservesTierReplacementAndEmptyLists(t *testing.T) {
 	// The same identity may appear in separate tiers, while each tier still
 	// retains its individual source applications for explanation replay.
 	if got := []Layer{assembly.current.applications[0].layer, assembly.current.applications[1].layer,
-		assembly.current.applications[2].layer, assembly.current.applications[3].layer}; !reflect.DeepEqual(got, []Layer{LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerBase}) {
+		assembly.current.applications[2].layer, assembly.current.applications[3].layer}; !reflect.DeepEqual(got, []Layer{LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerMain}) {
 		t.Fatalf("application layers = %#v", got)
 	}
 }
@@ -250,14 +250,14 @@ func TestLayeredUpsertExplanationFollowsStructuralOrder(t *testing.T) {
 			t.Fatal("structural upsert was misreported as a decoding reconciliation")
 		}
 	}
-	if last := explanation.History[len(explanation.History)-1]; last.After.YAML != explanation.Effective.YAML || last.Origin.Layer != LayerBase {
+	if last := explanation.History[len(explanation.History)-1]; last.After.YAML != explanation.Effective.YAML || last.Origin.Layer != LayerMain {
 		t.Fatalf("main contribution did not produce the effective collection: %#v", last)
 	}
 	removed, ok := loaded.Explain(Keys("records").Index(1).Key("removed"))
 	if !ok || len(removed.History) == 0 || removed.Effective.Present {
 		t.Fatalf("missing displaced field history: %#v, %v", removed, ok)
 	}
-	if last := removed.History[len(removed.History)-1]; last.After.Present || last.Origin.Layer != LayerBase || last.Origin.Operation == ChangeDecoding {
+	if last := removed.History[len(removed.History)-1]; last.After.Present || last.Origin.Layer != LayerMain || last.Origin.Operation == ChangeDecoding {
 		t.Fatalf("upsert removal attribution = %#v", last)
 	}
 }
@@ -650,7 +650,7 @@ func TestHistorySourceSequenceReplacementTracksMappedAndWholeSides(t *testing.T)
 	finalSequence, _ := documentAtPath(finalRoot, Keys("records"))
 	*finalSequence = *document.Clone(combined.value)
 	lowerOrigins := make(map[string]Origin)
-	collectOverlayOrigins(lowerOrigins, lowerRoot, Path{}, LayerBase)
+	collectOverlayOrigins(lowerOrigins, lowerRoot, Path{}, LayerMain)
 	higherOrigins := make(map[string]Origin)
 	collectOverlayOrigins(higherOrigins, higherRoot, Path{}, LayerOverlay)
 	assembly := newAssemblyAccumulator(&composedDocument{root: finalRoot, origins: lowerOrigins})
@@ -667,7 +667,7 @@ func TestHistorySourceSequenceReplacementTracksMappedAndWholeSides(t *testing.T)
 	}
 	higherApplication.sequencesByPath = indexSequenceHistoryContributions(higherApplication.sequences)
 	applications := []historyApplication{
-		{layer: LayerBase, value: lowerRoot}, higherApplication,
+		{layer: LayerMain, value: lowerRoot}, higherApplication,
 	}
 	builder := newHistoryBuilder()
 	builder.observeSourceEvidence(finalRoot, assembly.current.origins, applications, assembly.current.overlays)
@@ -728,7 +728,7 @@ func TestHistorySourceSequenceReplacementTracksMappedAndWholeSides(t *testing.T)
 	wholeFinalSequence, _ := documentAtPath(wholeFinal, Keys("records"))
 	*wholeFinalSequence = *document.Clone(whole.value)
 	wholeLowerOrigins := make(map[string]Origin)
-	collectOverlayOrigins(wholeLowerOrigins, wholeLower, Path{}, LayerBase)
+	collectOverlayOrigins(wholeLowerOrigins, wholeLower, Path{}, LayerMain)
 	wholeHigherOrigins := make(map[string]Origin)
 	collectOverlayOrigins(wholeHigherOrigins, wholeHigher, Path{}, LayerOverlay)
 	wholeAssembly := newAssemblyAccumulator(&composedDocument{root: wholeFinal, origins: wholeLowerOrigins})
@@ -743,7 +743,7 @@ func TestHistorySourceSequenceReplacementTracksMappedAndWholeSides(t *testing.T)
 	wholeHigherApplication.sequencesByPath = indexSequenceHistoryContributions(wholeHigherApplication.sequences)
 	wholeBuilder := newHistoryBuilder()
 	wholeBuilder.observeSourceEvidence(wholeFinal, wholeAssembly.current.origins, []historyApplication{
-		{layer: LayerBase, value: wholeLower}, wholeHigherApplication,
+		{layer: LayerMain, value: wholeLower}, wholeHigherApplication,
 	}, wholeAssembly.current.overlays)
 	wholeModel := wholeBuilder.freeze()
 	if len(wholeAssembly.current.overlays) != 1 {
diff --git a/internal/engine/history_source_test.go b/internal/engine/history_source_test.go
index 4ca03c0..620b062 100644
--- a/internal/engine/history_source_test.go
+++ b/internal/engine/history_source_test.go
@@ -17,10 +17,10 @@ func TestHistorySourceObservationsPreserveRawAndCompositeDefinitions(t *testing.
 	setHistoryDocumentSource(overlay, "overlay")
 
 	baseOrigins := make(map[string]Origin)
-	collectOverlayOrigins(baseOrigins, base, Path{}, LayerBase)
+	collectOverlayOrigins(baseOrigins, base, Path{}, LayerMain)
 	baseApplication := historyApplication{
-		layer: LayerBase, value: base,
-		origin: overlayOrigin(LayerBase, base.Location),
+		layer: LayerMain, value: base,
+		origin: overlayOrigin(LayerMain, base.Location),
 	}
 	composition := &composedDocument{
 		root: document.Clone(base), origins: baseOrigins,
@@ -91,7 +91,7 @@ func TestHistorySourceObservationsPreserveRawAndCompositeDefinitions(t *testing.
 	explanation, explained := inspection.Explain(Keys("section", "empty"))
 	var contributionSources []string
 	for _, change := range explanation.History {
-		if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerOverlay {
+		if change.Origin.Layer == LayerMain || change.Origin.Layer == LayerOverlay {
 			contributionSources = append(contributionSources, locationSourceIDs(change.Origin.Locations)...)
 		}
 	}
@@ -109,12 +109,12 @@ func TestHistorySourceReplacementTracksConcreteBeforeAndAfterShapes(t *testing.T
 	setHistoryDocumentSource(base, "base")
 	setHistoryDocumentSource(overlay, "overlay")
 	baseOrigins := make(map[string]Origin)
-	collectOverlayOrigins(baseOrigins, base, Path{}, LayerBase)
+	collectOverlayOrigins(baseOrigins, base, Path{}, LayerMain)
 	overlayOrigins := make(map[string]Origin)
 	collectOverlayOrigins(overlayOrigins, overlay, Path{}, LayerOverlay)
 	composition := &composedDocument{
 		root: document.Clone(base), origins: baseOrigins,
-		applications: []historyApplication{{layer: LayerBase, value: base, origin: overlayOrigin(LayerBase, base.Location)}},
+		applications: []historyApplication{{layer: LayerMain, value: base, origin: overlayOrigin(LayerMain, base.Location)}},
 	}
 	assembly := newAssemblyAccumulator(composition)
 	if diagnostics := assembly.applyOverlay(overlay, overlayOrigins, []historyApplication{{layer: LayerOverlay, value: overlay, origin: overlayOrigin(LayerOverlay, overlay.Location)}}, LayerOverlay); len(diagnostics) != 0 {
@@ -175,7 +175,7 @@ func TestOverlayReplacementCapturesIntroducedPathOrigins(t *testing.T) {
 	setHistoryDocumentSource(incoming, "overlay")
 	path := Keys("section")
 	origins := make(map[string]Origin)
-	collectOverlayOrigins(origins, existing, path, LayerBase)
+	collectOverlayOrigins(origins, existing, path, LayerMain)
 	incomingOrigins := make(map[string]Origin)
 	collectOverlayOrigins(incomingOrigins, incoming, path, LayerOverlay)
 	result := &composedDocument{
diff --git a/internal/engine/import_acquisition.go b/internal/engine/import_acquisition.go
index 5dbb5b1..c07a524 100644
--- a/internal/engine/import_acquisition.go
+++ b/internal/engine/import_acquisition.go
@@ -304,7 +304,7 @@ func (s *importTraversal) admitImportSource(canonical string, identity resourceI
 		kind = SourceKindFilesystem
 		label = s.access.label + "/" + canonical
 	}
-	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Destination: s.sourceDestination(destination), Layer: LayerBase, Kind: kind, ImportedFrom: &location}
+	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Destination: s.sourceDestination(destination), Layer: LayerMain, Kind: kind, ImportedFrom: &location}
 	if policy.placement == PlacementModeSequenceItem {
 		info.placementMode = PlacementModeSequenceItem
 	}
diff --git a/internal/engine/imports_test.go b/internal/engine/imports_test.go
index 8719d03..d155ac5 100644
--- a/internal/engine/imports_test.go
+++ b/internal/engine/imports_test.go
@@ -185,7 +185,7 @@ func TestLoadImportsFromProfilesDirectoryAsBaseInput(t *testing.T) {
 
 	result, err := testLoad[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
 	if err != nil || result.Config.Settings.Root != "imported" || len(result.Sources()) != 2 ||
-		result.Sources()[1].Layer != LayerBase {
+		result.Sources()[1].Layer != LayerMain {
 		t.Fatalf("profiles import = %#v, %v", result, err)
 	}
 }
@@ -573,7 +573,7 @@ func TestProjectNestedEvidenceUsesMappedSequenceItemIndex(t *testing.T) {
 	}
 	outer.sequencesByPath = indexSequenceHistoryContributions(outer.sequences)
 	nestedRoot := parseCompositionDocument(t, "nested-source", "settings: {value: nested}\n")
-	nestedOrigin := Origin{Layer: LayerBase, Locations: []Location{{SourceID: "nested-source"}}}
+	nestedOrigin := Origin{Layer: LayerMain, Locations: []Location{{SourceID: "nested-source"}}}
 	nested := &composedDocument{
 		root: nestedRoot, applications: []historyApplication{{value: nestedRoot}},
 		origins: map[string]Origin{"/settings/value": nestedOrigin},
diff --git a/internal/engine/inheritance_history_test.go b/internal/engine/inheritance_history_test.go
index c8c1ae1..04bce84 100644
--- a/internal/engine/inheritance_history_test.go
+++ b/internal/engine/inheritance_history_test.go
@@ -165,7 +165,7 @@ func TestInheritanceHistoryFollowsFinalParentAfterSourceOverlay(t *testing.T) {
 		t.Fatalf("overlaid parent contribution = %#v, %v", explanation, found)
 	}
 	parentHistory := inspection.Metadata().History(Keys("parent", "value"))
-	if len(parentHistory) < 2 || parentHistory[0].Origin.Layer != LayerBase ||
+	if len(parentHistory) < 2 || parentHistory[0].Origin.Layer != LayerMain ||
 		parentHistory[len(parentHistory)-1].Origin.Layer != LayerOverlay {
 		t.Fatalf("parent source history = %#v", parentHistory)
 	}
diff --git a/internal/engine/inheritance_origin.go b/internal/engine/inheritance_origin.go
index 0234674..f24a24e 100644
--- a/internal/engine/inheritance_origin.go
+++ b/internal/engine/inheritance_origin.go
@@ -12,7 +12,7 @@ func directInheritanceOrigins(node *document.Node, root Path, input map[string]O
 	walkDocument(node, root, func(path Path, value *document.Node) {
 		origin, found := input[path.String()]
 		if !found {
-			origin = Origin{Layer: LayerBase, Locations: []Location{publicLocation(value.Location)}}
+			origin = Origin{Layer: LayerMain, Locations: []Location{publicLocation(value.Location)}}
 		}
 		origin = cloneOrigin(origin)
 		if len(origin.contributors) == 0 {
diff --git a/internal/engine/inheritance_policy_index_test.go b/internal/engine/inheritance_policy_index_test.go
index da6fd57..cae34e9 100644
--- a/internal/engine/inheritance_policy_index_test.go
+++ b/internal/engine/inheritance_policy_index_test.go
@@ -104,7 +104,7 @@ func TestInheritanceWideIndependentYamlCollectionScopes(t *testing.T) {
 			t.Fatalf("independent scope %s composed incorrectly: %#v", name, group)
 		}
 		path := Keys("groups", name, "child", "items").Index(0).Key("value")
-		if info, exists := metadata.Lookup(path); !exists || !info.Explicit || info.Origin.Layer != LayerBase {
+		if info, exists := metadata.Lookup(path); !exists || !info.Explicit || info.Origin.Layer != LayerMain {
 			t.Fatalf("inherited destination lost source metadata at %s", path)
 		}
 	}
diff --git a/internal/engine/item_policy_test.go b/internal/engine/item_policy_test.go
index c9ca465..78e3ced 100644
--- a/internal/engine/item_policy_test.go
+++ b/internal/engine/item_policy_test.go
@@ -375,7 +375,7 @@ func TestOverlaySequenceTransfersIncomingItemPolicyOwnership(t *testing.T) {
 				root: lowerRoot, collectionPolicies: makePolicies("id"), origins: make(map[string]Origin),
 			})
 			if diagnostics := assembly.applyOverlayWithCollectionPolicy(
-				incomingRoot, nil, makePolicies("slug"), nil, LayerBase, registered, test.declarations,
+				incomingRoot, nil, makePolicies("slug"), nil, LayerMain, registered, test.declarations,
 			); len(diagnostics) != 0 {
 				t.Fatalf("overlay diagnostics = %#v", diagnostics)
 			}
@@ -412,7 +412,7 @@ func TestAncestorReplacementRequiresIdentityWhenRetainedModeReturns(t *testing.T
 		t.Helper()
 		if diagnostics := assembly.applyOverlayWithCollectionPolicy(
 			parseItemPolicyDocument(t, "remove", "catalog: null\n"), nil, collectionPolicyScope{}, nil,
-			LayerBase, sequencePolicy{}, nil,
+			LayerMain, sequencePolicy{}, nil,
 		); len(diagnostics) != 0 {
 			t.Fatalf("ancestor removal diagnostics = %#v", diagnostics)
 		}
@@ -427,7 +427,7 @@ func TestAncestorReplacementRequiresIdentityWhenRetainedModeReturns(t *testing.T
 		}
 		diagnostics := assembly.applyOverlayWithCollectionPolicy(
 			parseItemPolicyDocument(t, "reintroduction", "catalog:\n  prompts: [{id: restored, name: restored}]\n"), nil,
-			collectionPolicyScope{}, nil, LayerBase, sequencePolicy{}, nil,
+			collectionPolicyScope{}, nil, LayerMain, sequencePolicy{}, nil,
 		)
 		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid {
 			t.Fatalf("append without renewed identity diagnostics = %#v", diagnostics)
@@ -442,7 +442,7 @@ func TestAncestorReplacementRequiresIdentityWhenRetainedModeReturns(t *testing.T
 		}
 		diagnostics := assembly.applyOverlayWithCollectionPolicy(
 			parseItemPolicyDocument(t, "keyed", "catalog:\n  prompts: [{name: next}]\n"), nil,
-			collectionPolicyScope{}, nil, LayerBase, sequencePolicy{}, []sequenceDeclarationSpec{declaration},
+			collectionPolicyScope{}, nil, LayerMain, sequencePolicy{}, []sequenceDeclarationSpec{declaration},
 		)
 		if len(diagnostics) != 0 {
 			t.Fatalf("renewed identity diagnostics = %#v", diagnostics)
diff --git a/internal/engine/load_input.go b/internal/engine/load_input.go
index 3b39564..65db612 100644
--- a/internal/engine/load_input.go
+++ b/internal/engine/load_input.go
@@ -132,7 +132,7 @@ func applyInputLayers(source Source, filesystem *FilesystemContext, overlay stri
 func completeOrdinaryResourceContexts(input assembledInput, main loadInput) (assembledInput, []Diagnostic) {
 	baseSources := make([]SourceInfo, 0, len(input.sources))
 	for _, source := range input.sources {
-		if source.Layer == LayerBase {
+		if source.Layer == LayerMain {
 			baseSources = append(baseSources, source)
 		}
 	}
diff --git a/internal/engine/load_request.go b/internal/engine/load_request.go
index b2d1801..dea932e 100644
--- a/internal/engine/load_request.go
+++ b/internal/engine/load_request.go
@@ -152,7 +152,7 @@ func assembleLayeredInput[T any](request loadRequest[T], input assembledInput, p
 	if mainComposition != nil {
 		input.versionState.locateMainVersion(mainComposition.root)
 		operations, diagnostics := applyCompletedTier(
-			&assembly, mainComposition, mainComposition.collectionPolicies, LayerBase, policy.sequencePolicy,
+			&assembly, mainComposition, mainComposition.collectionPolicies, LayerMain, policy.sequencePolicy,
 			input.sources[:mainSourceCount], mainEvidence,
 		)
 		if len(diagnostics) != 0 {
diff --git a/internal/engine/load_request_test.go b/internal/engine/load_request_test.go
index 1b9053d..d1c4c24 100644
--- a/internal/engine/load_request_test.go
+++ b/internal/engine/load_request_test.go
@@ -294,7 +294,7 @@ func TestGroupedLoadPrecedenceSourceOrderAndSingleLifecycle(t *testing.T) {
 	if len(sources) != 7 {
 		t.Fatalf("sources = %#v", sources)
 	}
-	wantLayers := []Layer{LayerBase, LayerBase, LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerOverlay, LayerCLI}
+	wantLayers := []Layer{LayerMain, LayerMain, LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerOverlay, LayerCLI}
 	wantIndexes := []int{0, 0, 1, 1, 2, 0, 0}
 	for index, source := range sources {
 		if source.ID != "source-"+string(rune('1'+index)) || source.Layer != wantLayers[index] || source.UnderlayIndex != wantIndexes[index] {
@@ -315,7 +315,7 @@ func TestGroupedLoadPrecedenceSourceOrderAndSingleLifecycle(t *testing.T) {
 			layers = append(layers, change.Origin.Layer)
 		}
 	}
-	if !reflect.DeepEqual(layers, []Layer{LayerUnderlay, LayerBase, LayerOverlay, LayerCLI}) {
+	if !reflect.DeepEqual(layers, []Layer{LayerUnderlay, LayerMain, LayerOverlay, LayerCLI}) {
 		t.Fatalf("history layers = %#v", layers)
 	}
 }
diff --git a/internal/engine/load_test.go b/internal/engine/load_test.go
index dc49ca0..4809e00 100644
--- a/internal/engine/load_test.go
+++ b/internal/engine/load_test.go
@@ -523,17 +523,17 @@ func TestLoadMetadataReflectsDefaultsInputAndDetachedRecords(t *testing.T) {
 		t.Fatalf("kept = %#v, %v", kept, ok)
 	}
 	set, ok := metadata.Lookup(Keys("nested", "set"))
-	if !ok || !set.Explicit || set.Origin.Layer != LayerBase || len(set.Origin.Locations) != 1 {
+	if !ok || !set.Explicit || set.Origin.Layer != LayerMain || len(set.Origin.Locations) != 1 {
 		t.Fatalf("set = %#v, %v", set, ok)
 	}
 	nullable, ok := metadata.Lookup(Keys("nullable"))
-	if !ok || !nullable.Explicit || !nullable.Null || nullable.Origin.Layer != LayerBase {
+	if !ok || !nullable.Explicit || !nullable.Null || nullable.Origin.Layer != LayerMain {
 		t.Fatalf("nullable = %#v, %v", nullable, ok)
 	}
 	if metadata.IsExplicit(Keys("nested", "kept")) || !metadata.IsExplicit(Keys("nested", "set")) {
 		t.Fatal("explicit presence disagrees with lookup")
 	}
-	if history := metadata.History(Keys("nested", "set")); len(history) != 2 || history[0].Origin.Layer != LayerDefault || history[1].Origin.Layer != LayerBase || len(history[1].Previous) != 1 {
+	if history := metadata.History(Keys("nested", "set")); len(history) != 2 || history[0].Origin.Layer != LayerDefault || history[1].Origin.Layer != LayerMain || len(history[1].Previous) != 1 {
 		t.Fatalf("history = %#v", history)
 	}
 
diff --git a/internal/engine/metadata_locations_test.go b/internal/engine/metadata_locations_test.go
index f45721b..7746309 100644
--- a/internal/engine/metadata_locations_test.go
+++ b/internal/engine/metadata_locations_test.go
@@ -117,7 +117,7 @@ func TestHistoricalLocationSelectionSeparatesNumericKeyAndIndex(t *testing.T) {
 	}}}
 	builder := newHistoryBuilder()
 	builder.observeSourceEvidence(sequenceRoot, nil, []historyApplication{
-		{layer: LayerBase, value: mappingRoot},
+		{layer: LayerMain, value: mappingRoot},
 		{layer: LayerOverlay, value: sequenceRoot},
 	}, nil)
 	model := builder.freeze()
diff --git a/internal/engine/metadata_test.go b/internal/engine/metadata_test.go
index c318dc5..22c1a3f 100644
--- a/internal/engine/metadata_test.go
+++ b/internal/engine/metadata_test.go
@@ -146,7 +146,7 @@ func TestHistoricalPathsExposeHiddenBaseOwnershipWithoutResolvingBaseSecrets(t *
 				found = true
 				owners := map[string]bool{}
 				for _, change := range metadata.History(path) {
-					if change.Origin.Layer == LayerBase {
+					if change.Origin.Layer == LayerMain {
 						for _, location := range change.Origin.Locations {
 							owners[location.SourceID] = true
 						}
@@ -188,7 +188,7 @@ func TestMetadataQueriesReturnDetachedRecords(t *testing.T) {
 			path.String(): {
 				Explicit: true,
 				Origin: Origin{
-					Layer:     LayerBase,
+					Layer:     LayerMain,
 					Locations: []Location{{SourceID: "source-1", Line: 2, Column: 3}},
 				},
 			},
@@ -246,7 +246,7 @@ func TestResolutionInputKeepsDetachedEntryMetadata(t *testing.T) {
 	}
 	metadata := metadataFromDecoded(decoded)
 	input := newResolutionInput(metadata, decoded.final, []SourceInfo{{
-		ID: "source-1", Label: "decode.yml", Layer: LayerBase,
+		ID: "source-1", Label: "decode.yml", Layer: LayerMain,
 		ImportedFrom: &Location{SourceID: "parent", Line: 1, Column: 1},
 	}})
 
diff --git a/internal/engine/override_context.go b/internal/engine/override_context.go
index 83f7f82..939eaee 100644
--- a/internal/engine/override_context.go
+++ b/internal/engine/override_context.go
@@ -146,7 +146,7 @@ func structuralHistoryApplications(input assembledInput) []historyApplication {
 	if input.document == nil {
 		return nil
 	}
-	return []historyApplication{{layer: LayerBase, value: input.document}}
+	return []historyApplication{{layer: LayerMain, value: input.document}}
 }
 
 // documentWithMetadataLocations attaches effective origin coordinates to a
diff --git a/internal/engine/override_context_test.go b/internal/engine/override_context_test.go
index e067439..e9eb225 100644
--- a/internal/engine/override_context_test.go
+++ b/internal/engine/override_context_test.go
@@ -33,7 +33,7 @@ func TestPostDecodeOverrideContextOrdersDecodeCLIAndResolutionEvidence(t *testin
 	if len(issues) != 0 {
 		t.Fatalf("decodeConfigurationWithContext() issues = %#v", issues)
 	}
-	sources := []SourceInfo{{ID: "source-1", Label: "main input", Layer: LayerBase, Kind: SourceKindMapping}}
+	sources := []SourceInfo{{ID: "source-1", Label: "main input", Layer: LayerMain, Kind: SourceKindMapping}}
 	contexts := map[string]ResourceContext{"source-1": {BaseDir: "/config"}}
 	input := assembledInput{document: structural, sources: sources, contexts: contexts}
 	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: structural}, sources, []Override{
@@ -114,7 +114,7 @@ func TestPostDecodeOverrideContextOrdersDecodeCLIAndResolutionEvidence(t *testin
 		t.Fatal("Explain(rate) did not return a history")
 	}
 	if len(rateExplanation.History) != 4 || rateExplanation.History[0].Origin.Layer != LayerDefault ||
-		rateExplanation.History[1].Origin.Layer != LayerBase || rateExplanation.History[2].Origin.Operation != ChangeDecoding ||
+		rateExplanation.History[1].Origin.Layer != LayerMain || rateExplanation.History[2].Origin.Operation != ChangeDecoding ||
 		rateExplanation.History[3].Origin.Layer != LayerCLI {
 		t.Fatalf("rate history order = %#v", rateExplanation.History)
 	}
diff --git a/internal/engine/override_history_order_test.go b/internal/engine/override_history_order_test.go
index 5178793..ec7ff85 100644
--- a/internal/engine/override_history_order_test.go
+++ b/internal/engine/override_history_order_test.go
@@ -62,7 +62,7 @@ func TestCLIHistoryFollowsInheritanceAndPrecedesResolution(t *testing.T) {
 						t.Fatal("inheritance reported after CLI")
 					}
 					inherited = true
-				} else if change.Origin.Layer == LayerBase && cli {
+				} else if change.Origin.Layer == LayerMain && cli {
 					t.Fatal("source definition reappeared after CLI")
 				}
 				if change.Origin.Layer == LayerCLI {
diff --git a/internal/engine/override_lifecycle_test.go b/internal/engine/override_lifecycle_test.go
index bdc1f44..c0a7f2f 100644
--- a/internal/engine/override_lifecycle_test.go
+++ b/internal/engine/override_lifecycle_test.go
@@ -29,7 +29,7 @@ func TestNestedCLIOverridePreservesRetainedInputPresence(t *testing.T) {
 			definition := Definition[config]{Resolve: func(value *config, resolution *Resolution) error {
 				for _, key := range []string{"count", "zero", "enabled"} {
 					info, ok := resolution.InputInfo(Keys("settings", "nested", key))
-					if !ok || !info.Explicit || info.Origin.Layer != LayerBase {
+					if !ok || !info.Explicit || info.Origin.Layer != LayerMain {
 						t.Errorf("retained %s input info = %#v, %v", key, info, ok)
 					}
 				}
@@ -54,7 +54,7 @@ func TestNestedCLIOverridePreservesRetainedInputPresence(t *testing.T) {
 			}
 			for _, key := range []string{"count", "zero", "enabled"} {
 				info, ok := result.Metadata().Lookup(Keys("settings", "nested", key))
-				if !ok || !info.Explicit || info.Origin.Layer != LayerBase || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" {
+				if !ok || !info.Explicit || info.Origin.Layer != LayerMain || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" {
 					t.Errorf("retained %s metadata = %#v, %v", key, info, ok)
 				}
 			}
@@ -112,7 +112,7 @@ func TestPostDecodeOverrideLifecycleMatchesOrdinaryAndLayeredLoads(t *testing.T)
 		if result.Config.Count != 5 || result.Config.Derived != 8 {
 			t.Errorf("%s resolved configuration = %#v", name, result.Config)
 		}
-		if got := historyLayers(result.Metadata().History(Keys("count"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerCLI, LayerResolution}) {
+		if got := historyLayers(result.Metadata().History(Keys("count"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerMain, LayerCLI, LayerResolution}) {
 			t.Errorf("%s count history layers = %#v", name, got)
 		}
 		info, ok := result.Metadata().Lookup(Keys("count"))
diff --git a/internal/engine/override_metadata_test.go b/internal/engine/override_metadata_test.go
index 51d4902..7bed671 100644
--- a/internal/engine/override_metadata_test.go
+++ b/internal/engine/override_metadata_test.go
@@ -52,12 +52,12 @@ func TestBroadPartialCLIOverrideRetainsUnchangedMetadata(t *testing.T) {
 		value := result.value.Interface().(config).Records[name]
 		for _, key := range []string{"zero", "enabled"} {
 			pointer := path.Key(key).String()
-			if _, explicit := result.presence[pointer]; !explicit || result.origins[pointer].Layer != LayerBase {
+			if _, explicit := result.presence[pointer]; !explicit || result.origins[pointer].Layer != LayerMain {
 				t.Fatalf("retained explicit field lost metadata at %s", pointer)
 			}
 		}
 		if index%2 != 0 {
-			if value.Count != 9 || len(value.Items) != 1 || value.Maybe == nil || result.origins[path.Key("count").String()].Layer != LayerBase {
+			if value.Count != 9 || len(value.Items) != 1 || value.Maybe == nil || result.origins[path.Key("count").String()].Layer != LayerMain {
 				t.Fatalf("omitted record changed at %s", path)
 			}
 			continue
diff --git a/internal/engine/override_test.go b/internal/engine/override_test.go
index ffe47d2..82a552d 100644
--- a/internal/engine/override_test.go
+++ b/internal/engine/override_test.go
@@ -95,7 +95,7 @@ func TestOverridesRunAtHighestInputPrecedenceBeforeResolutionAndValidation(t *te
 		t.Fatalf("effective settings = %#v", result.Config.Settings)
 	}
 	sources := result.Sources()
-	if len(sources) != 3+len(overrides) || sources[0].Layer != LayerBase || sources[1].Layer != LayerBase || sources[2].Layer != LayerOverlay {
+	if len(sources) != 3+len(overrides) || sources[0].Layer != LayerMain || sources[1].Layer != LayerMain || sources[2].Layer != LayerOverlay {
 		t.Fatalf("sources = %#v", sources)
 	}
 	for index, override := range overrides {
@@ -104,7 +104,7 @@ func TestOverridesRunAtHighestInputPrecedenceBeforeResolutionAndValidation(t *te
 		}
 	}
 	history := result.Metadata().History(Keys("settings", "target"))
-	wantLayers := []Layer{LayerDefault, LayerBase, LayerOverlay, LayerCLI, LayerResolution}
+	wantLayers := []Layer{LayerDefault, LayerMain, LayerOverlay, LayerCLI, LayerResolution}
 	if got := historyLayers(history); !reflect.DeepEqual(got, wantLayers) {
 		t.Fatalf("target history layers = %#v, want %#v; history %#v", got, wantLayers, history)
 	}
@@ -392,7 +392,7 @@ func TestOverrideEqualReplacementStillAppearsInHistory(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if got := historyLayers(result.Metadata().History(Keys("value"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerCLI}) {
+	if got := historyLayers(result.Metadata().History(Keys("value"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerMain, LayerCLI}) {
 		t.Fatalf("equal override history = %#v", result.Metadata().History(Keys("value")))
 	}
 }
@@ -453,7 +453,7 @@ func TestPreparedOverrideBatchDetachesInputsAndAllocatesSourcesInOrder(t *testin
 	if len(diagnostics) != 0 {
 		t.Fatal(diagnostics)
 	}
-	sources := []SourceInfo{{ID: "source-1", Label: "base", Layer: LayerBase, Kind: SourceKindMapping}}
+	sources := []SourceInfo{{ID: "source-1", Label: "base", Layer: LayerMain, Kind: SourceKindMapping}}
 	firstValue := map[string]any{"nested": map[string]any{"name": "original"}}
 	secondValue := []any{map[string]any{"enabled": true}}
 	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{
@@ -630,7 +630,7 @@ func TestApplyDecodedOverridesUsesSchemaAndPreservesTypedOwnership(t *testing.T)
 	}
 	if result.presence["/items/0/name"].location.SourceID != batch.overrides[2].source.ID ||
 		result.presence["/nested"].location.SourceID != "source-1" ||
-		result.origins["/items/0/name"].Layer != LayerCLI || result.origins["/nested/name"].Layer != LayerBase ||
+		result.origins["/items/0/name"].Layer != LayerCLI || result.origins["/nested/name"].Layer != LayerMain ||
 		result.origins["/nested/enabled"].Layer != LayerCLI || result.origins["/labels/default/name"].Layer != LayerDefault {
 		t.Fatalf("presence/origins = %#v / %#v", result.presence, result.origins)
 	}
diff --git a/internal/engine/provenance_test.go b/internal/engine/provenance_test.go
index f013abc..a02fbf6 100644
--- a/internal/engine/provenance_test.go
+++ b/internal/engine/provenance_test.go
@@ -99,7 +99,7 @@ empty: {}
 	nullable, _ := metadata.Lookup(Keys("nullable"))
 	empty, _ := metadata.Lookup(Keys("empty"))
 	firstValue, _ := metadata.Lookup(Keys("values").Index(0))
-	if !equal.Explicit || equal.Origin.Layer != LayerBase || derived.Explicit || derived.Origin.Operation != ChangeDerivedDefault ||
+	if !equal.Explicit || equal.Origin.Layer != LayerMain || derived.Explicit || derived.Origin.Operation != ChangeDerivedDefault ||
 		!nullable.Explicit || !nullable.Null || nullable.Origin.Layer != LayerOverlay ||
 		!empty.Explicit || empty.Origin.Layer != LayerOverlay || len(empty.Origin.Locations) != 2 ||
 		!firstValue.Explicit || firstValue.Origin.Operation != ChangeNormalization {
@@ -116,14 +116,14 @@ empty: {}
 	}
 
 	assertHistoryLayersAndPaths(t, metadata.History(Keys("values").Index(1)),
-		[]Layer{LayerDefault, LayerBase, LayerOverlay, LayerResolution},
+		[]Layer{LayerDefault, LayerMain, LayerOverlay, LayerResolution},
 		[]string{"/values/1", "/values/1", "/values", "/values"})
 	assertHistoryLayersAndPaths(t, metadata.History(Keys("value", "child")),
-		[]Layer{LayerDefault, LayerBase, LayerOverlay},
+		[]Layer{LayerDefault, LayerMain, LayerOverlay},
 		[]string{"/value/child", "/value/child", "/value"})
 	removedHistory := metadata.History(Keys("items", "remove", "value"))
 	assertHistoryLayersAndPaths(t, removedHistory,
-		[]Layer{LayerBase, LayerResolution},
+		[]Layer{LayerMain, LayerResolution},
 		[]string{"/items/remove/value", "/items"})
 	if !reflect.DeepEqual(removedHistory[1].Reasons, []string{"removed disabled items"}) {
 		t.Fatalf("removed history = %#v", removedHistory)
diff --git a/internal/engine/records.go b/internal/engine/records.go
index 13ff9a9..f9ecfbe 100644
--- a/internal/engine/records.go
+++ b/internal/engine/records.go
@@ -5,7 +5,7 @@ type Layer string
 
 const (
 	LayerDefault    Layer = "default"
-	LayerBase       Layer = "base"
+	LayerMain       Layer = "main"
 	LayerUnderlay   Layer = "underlay"
 	LayerOverlay    Layer = "overlay"
 	LayerCLI        Layer = "cli"
diff --git a/internal/engine/resolution_index_test.go b/internal/engine/resolution_index_test.go
index 100fffd..267c306 100644
--- a/internal/engine/resolution_index_test.go
+++ b/internal/engine/resolution_index_test.go
@@ -208,7 +208,7 @@ func TestDocumentLookupMatchesPathScan(t *testing.T) {
 }
 
 func TestApplyResolutionMetadataPreservesOverlappingDeltaOrder(t *testing.T) {
-	oldOrigin := Origin{Layer: LayerBase}
+	oldOrigin := Origin{Layer: LayerMain}
 	parentOrigin := Origin{Layer: LayerResolution, Operation: ChangeNormalization}
 	childOrigin := Origin{Layer: LayerResolution, Operation: ChangeDerivedDefault}
 	beforeLeaf := &document.Node{Kind: document.String, Text: "before"}
diff --git a/internal/engine/selected_overlay_test.go b/internal/engine/selected_overlay_test.go
index 5f17644..fbc5b01 100644
--- a/internal/engine/selected_overlay_test.go
+++ b/internal/engine/selected_overlay_test.go
@@ -153,7 +153,7 @@ value: scalar
 	}
 	changed, _ := result.Metadata().Lookup(Keys("settings", "changed"))
 	kept, _ := result.Metadata().Lookup(Keys("settings", "kept"))
-	if changed.Origin.Layer != LayerOverlay || changed.Origin.Locations[0].SourceID != "source-3" || kept.Origin.Layer != LayerBase {
+	if changed.Origin.Layer != LayerOverlay || changed.Origin.Locations[0].SourceID != "source-3" || kept.Origin.Layer != LayerMain {
 		t.Fatalf("origins = changed %#v kept %#v", changed, kept)
 	}
 	if _, ok := result.Metadata().Lookup(Keys("value", "child")); ok {
@@ -248,7 +248,7 @@ items: [{id: c, value: replacement}]
 			if test.mode == "append" {
 				base, _ := result.Metadata().Lookup(Keys("items").Index(0).Key("value"))
 				added, _ := result.Metadata().Lookup(Keys("items").Index(2).Key("value"))
-				if base.Origin.Layer != LayerBase || added.Origin.Layer != LayerOverlay {
+				if base.Origin.Layer != LayerMain || added.Origin.Layer != LayerOverlay {
 					t.Fatalf("append origins = base %#v added %#v", base.Origin, added.Origin)
 				}
 				history := result.Metadata().History(Keys("items").Index(2).Key("value"))
diff --git a/internal/engine/source.go b/internal/engine/source.go
index e66f57c..a4cc6c7 100644
--- a/internal/engine/source.go
+++ b/internal/engine/source.go
@@ -222,7 +222,7 @@ func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead
 	if limit < 0 {
 		limit = 0
 	}
-	info := &SourceInfo{ID: sourceID, Label: source.label, Layer: LayerBase, Kind: sourceSourceKind(source.kind)}
+	info := &SourceInfo{ID: sourceID, Label: source.label, Layer: LayerMain, Kind: sourceSourceKind(source.kind)}
 
 	var input io.Reader
 	switch source.kind {
@@ -285,7 +285,7 @@ func mappingSourceInfo(source Source, sourceID string) *SourceInfo {
 	if !validOverrideLabel(label) {
 		label = "mapping"
 	}
-	return &SourceInfo{ID: sourceID, Label: label, Layer: LayerBase, Kind: SourceKindMapping}
+	return &SourceInfo{ID: sourceID, Label: label, Layer: LayerMain, Kind: SourceKindMapping}
 }
 
 func sourceConstructorDiagnostic(source Source) *Diagnostic {
diff --git a/internal/engine/validation_test.go b/internal/engine/validation_test.go
index f1651de..cb262a5 100644
--- a/internal/engine/validation_test.go
+++ b/internal/engine/validation_test.go
@@ -208,7 +208,7 @@ func TestCompleteSingleSourceLifecycleKeepsMetadataAgreement(t *testing.T) {
 	input, _ := result.Metadata().Lookup(Keys("input"))
 	derived, _ := result.Metadata().Lookup(Keys("derived"))
 	history := result.Metadata().History(Keys("derived"))
-	if result.Config != (config{Input: 3, Derived: 6}) || !input.Explicit || input.Origin.Layer != LayerBase || derived.Explicit || derived.Origin.Operation != ChangeDerivedDefault || history[len(history)-1].Dependencies[0].String() != "/input" {
+	if result.Config != (config{Input: 3, Derived: 6}) || !input.Explicit || input.Origin.Layer != LayerMain || derived.Explicit || derived.Origin.Operation != ChangeDerivedDefault || history[len(history)-1].Dependencies[0].String() != "/input" {
 		t.Fatalf("config/metadata = %#v / %#v / %#v / %#v", result.Config, input, derived, history)
 	}
 }
diff --git a/records.go b/records.go
index b1045da..5f4f395 100644
--- a/records.go
+++ b/records.go
@@ -5,7 +5,7 @@ type Layer string
 
 const (
 	LayerDefault    Layer = "default"
-	LayerBase       Layer = "base"
+	LayerMain       Layer = "main"
 	LayerUnderlay   Layer = "underlay"
 	LayerOverlay    Layer = "overlay"
 	LayerCLI        Layer = "cli"
diff --git a/records_test.go b/records_test.go
index 2550e42..afafb9f 100644
--- a/records_test.go
+++ b/records_test.go
@@ -8,6 +8,12 @@ import (
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
 )
 
+func TestLayerMainString(t *testing.T) {
+	if got := string(LayerMain); got != "main" {
+		t.Fatalf("main layer string = %q", got)
+	}
+}
+
 func TestOverlayPublicValues(t *testing.T) {
 	if LayerOverlay != "overlay" {
 		t.Fatalf("LayerOverlay = %q", LayerOverlay)