The pinnacle of iOS application design is the Universal Application. But what does that mean? A universal application is a single binary that will work optimally for all iOS platforms. When the iPad first became available, application developers made iPhone and iPad versions of their application. That is quite inefficient for developers as well as maddening for the consumer. Buyers didn’t want to buy 2 versions of the same application so they could use it on all of their iOS devices.
The goal of any universal iOS application is a very stringent Model-View-Controller architecture. This means that there is minimal differences between the iPhone and iPad versions. All of the logic of the application should be sources from the same Objective-C classes. Done correctly from the beginning, there is minimal extra expense in making the application universal