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web/core/modules/migrate_drupal/config/install/migrate_drupal.settings.yml

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+# Migrations with any of these tags will raise an exception if their source
+# plugin is missing the source_module property in their annotation.
+enforce_source_module_tags:
+  - Drupal 6
+  - Drupal 7
+# Migrations with any of these tags will not be derived and executed with the
+# other migrations. They will be derived and executed after the migrations on
+# which they depend have been successfully executed.
+follow_up_migration_tags:
+  - Follow-up migration

web/core/modules/migrate_drupal/config/schema/migrate_drupal.schema.yml

@@ -0,0 +1,16 @@
+migrate_drupal.settings:
+  type: config_object
+  label: 'Migrate Drupal settings'
+  mapping:
+    enforce_source_module_tags:
+      type: sequence
+      label: 'source_module enforcement tags'
+      sequence:
+        type: string
+        label: 'Tag'
+    follow_up_migration_tags:
+      type: sequence
+      label: 'Follow-up migration tags'
+      sequence:
+        type: string
+        label: 'Tag'