@interface Tab2AppDelegate : NSObject
This class implements two protocols, the usual UIApplicationDelegate and a new acquaintance called UITabBarControllerDelegate. An educated guess whould be that the new protocol is used for "receiving messages" from a UITabBarController.
@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet UITabBarController *tabBarController;