Does it have to do with mapping between different data models? To which other Core Data classes is this related? It doesn't look too obvious to me where this belongs to. Merging data models? Or is that just a stand-alone thing?
When would I use NSPropertyMapping for example?
NSPropertyMapping is used when migrating data models to map a property in a source entity (older data model) to a property in a destination entity (newer data model).
NSPropertyMapping objects are created alongside NSEntityMapping objects as part of a NSMappingModel when creating a mapping model in Xcode: File > New File > iPhone OS / Mac OS > Resource > Mapping Model.