You can assign Knobs A and B to control the Gate, Velocity, Type or Unit Multiply. Knobs A and B are global, system wide assignments. Knobs 1 and 2 are done in the Voice Edit Control Sets are specific assignments for that Voice.

Assign the Knobs is easy: In Utility Mode Press F4 Controller Assignments Press SF2 - Assign - This is where you can assign the A and B Knobs to different functions.

While we are there, check out SF1 - Arp: Here you can assign the CC# for arpeggio on/off and the CC3 for Arp Hold on/off.