From Xojo Documentation
Set to the default iOSScreen to use when the app is launched on an iPad-sized device.
If no DefaultiPadScreen is specified, the app will still run on an iPad but will use the DefaultiPhoneScreen, running it in "scaled" mode.
One or both of DefaultiPhoneScreen and DefaultiPadScreen must be specified.
By specifying separate screens for DefaultiPhoneScreen and DefaultiPadScreen, you can have completely different layouts for each type of device.