The case attaches to the micro USB (Apple users can suck it) and is usable by apps.
It's main function would be as a keyboard so that people can touch-type with 4 fingers on the back of the phone, leaving the screen free. There would be a training mode that displays the keyboard on the screen to learn. Function keys would be round the edge.
Other applications would be up to the imaginations of app developers, such as for a game controller or a musical instrument.-- marklar, May 24 2018random, halfbakery