Unfortunately it is to the right on the marketing material under the following text...
MainStage supports your addiction to knobs, buttons, and faders. It can connect with just about any external MIDI device on the planet. And the interface is simplicity itself. Just assign a physical control to an onscreen control, and it’s connected forever (or until you change your mind). No more assigning knobs over and over for each preset. And down the road, if your hardware fails when you’re about to go on stage, it’s quick and easy to swap it out.
MainStage 3 also features pickup modes that make fixed controller knobs behave more like the endless encoder knobs found on high-end gear. If your control surface can receive status information from software, MainStage will send MIDI messages to keep your control displays in perfect sync.
Was trying to do something similar with my Maschine but it doesn't seem obvious,