What does this mean?

“Fully assignable controls for mod wheel, pitch wheel, pads, knobs, faders, buttons, and transport control including XY pad with Mackie Control and HUI modes”

Description for M-Audio Code
http://www.musiciansfriend.com/keyboards-midi/m-audio-code-61-usb-midi-keyboard-controller

I can assume, Mackie Control is specific to a Mackie external mixer (or is it a more general protocol/language thing?), but what is HUI?

mackie control is a protocol and so is Hui (human user interface)

In my experience, Hui is more generic and mackie control protocol is more tailored to the platform (software).