The HUI MIDI mapping protocol is a proprietary communications protocol for interfacing between a hardware audio control surface and digital audio workstation (DAW) software. The protocol allows a DAW and a connected hardware control surface to exchange MIDI signals that synchronize the states of their (virtual) sliders, buttons, wheels, and displays. The user can write console automation which can then be seen in the DAW.
This protocol was created jointly by Mackie and Digidesign in 1997 for Mackie HUI, the first hardware control surface for Digidesign’s Pro Tools. It was also the protocol used in the Mackie Baby HUI released much later.
It is now used very widely. Many hardware controllers (Tascam FW-1082, Novation Launchkey, Mackie Control Universal, Yamaha 01X, Alesis MasterControl, SSL AWS900, etc.) implement this protocol. Similarly many types of DAW software implement the HUI protocol, making them cross-compatible with the HUI-compatible hardware controllers.
While HUI is still supported on the hardware side, non-Digi software has moved on to more open MCU and other MIDI control protocols as Digidesign (now Avid) has made extensive controllers with proprietary protocols that have much more rich capabilities but are part of a closed system.