This feature has yet to be added to the Unreal Documentation. Epic Staff have added content about this feature on the Epic Games Community portal here: Unreal Stage App.
Unreal Stage App
An application designed to utilize a tablet as a wireless control panel for specific Unreal Engine features in a physical space.
Learn to use this Experimental feature, but use caution when shipping with it.
Ask questions and help your peers Developer Forums
Write your own tutorials or read those from others Learning Library