Note, saving the child context applies the changes to the parent context. Saving the parent context persists the changes

When Core Data fetches an object from a persistent store, it takes a snapshot of its state. A snapshot is a dictionary of an object’s persistent properties—typically all its attributes and the global IDs of any objects to which it has a to-one relationship

the context owns the queue and manages all the details for you

performBlock: will add the block to the backing queue and schedule it to run on its own thread. The block will return immediately. You might use this for long persist operations to the backing store. performBlockAndWait: will also add the block to the backing queue and schedule it to run on its own thread. However, the block will not return until the block is finished executing. If you can't move on until you know whether the operation was successful, then this is your choice

NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType. If you do anything with one of these context types, such as setting the persistent store or saving changes, you do it in a block