Restoration Identifiers and Classes
When the application’s state is being saved, the window’s rootViewController is asked for its restorationIdentifier. If it has a restorationIdentifier, it is asked to save (or encode) its state. Then it is responsible for processing its child view controllers in the same way, and they, in turn, pass the torch to their child view controllers. If any view controller in the hierarchy does not have a restorationIdentifier, however, it (and its child view controllers, whether or not they have restorationIdentifiers) will be excluded from the saved state.
Figure 24.2 Restoration identifiers