How iPhone 5 + iOS6 will decide if an app must be run in letterbox mode

How the iOS6 decides if the app must be run in letterbox/compatibility mode on iPhone 5? Is this a build settings parameter (like the "Targeted device family")? Or all apps build against the latest SDK MUST support iPhone 5 screen size? Or the app will be run in letterbox mode is there is not a 1136x640 Default.png splashscreen ? Or what else?

The app will run in letterbox mode if there is no 1136x640 Default.png splashscreen.

The image must be called [email protected] (if your splashscreen is Default.png in your Info.plist)

You will need to add IOS8 specific splash screen too. In my case i had 1136x640 Default.png splashscreen but you need to select launch screen and select orientation as shown below

This worked for me. Hope this helps :)