Generally speaking a digital piano is designed for home use. It probably has elements of a piece of furniture about it, will look stylish in your living room and needs no other gear to hear the sound. A stage piano, while it is also a digital piano, will be designed to survive the rigours of being on the road. It has to be easy to set up, portable and robust. So they will dispense with the frame and music stand and focus on the keybed, controls and getting the sound routed to the P.A. You won’t find any internal speakers.