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A Core Data mapping model describes the transformations that are required to migrate data described by a source managed object model to the schema described by a destination model. When you make a new version of a managed object model, you need to migrate persistent stores from the old schema to the new. For simple changes to the model, Core Data can infer the required mapping (as described in Core Data Model Versioning and Data Migration Programming Guide). For more complex changes, you need to provide a mapping model describe how to perform the migration.