These values correspond to NSAttributeType NSAttributeDescription . They are similar to the strings returned by the NSValue objCType method inherited by NSNumber . The main database must know how to encode / decode the value in persistent storage, as well as the init method to call when creating the NSNumber object. Types in NSEntityDescription provide the ability to dynamically create the correct object from storage and save them again. Generally, you don't need to worry about the details of storing NSValue classes and subclasses. These details are hidden by purpose. If you are implementing your own NSAtomicStore, then you will need to know. For more information, see Atomic Store programming topics .