What happens when the “create document-based application” option is chosen in Xcode?

When going through some sample code, notice that setting the main Window Title in IB has no effect when the "create document-based application" option is chosen. The Window would come out with "Untitled" as its title. And we need to override - (NSString *) displayName {} in MyDocument : NSDocument inorder to set the window title. Just wish that some experts in xcode can explain why this is so. That is, what is added to the system when we check the "create document-based application" option when the project is initially generated ?