M1, M2, M3, and M4 knobs are modulation sources that appear the MOD Matrix
( see Mode Matrix Section [p.38] ) on rows M, N, O, and P. They're designed to be
convenient performance controls.
Each MACRO knob can modulate multiple MatrixBrute parameters, and they all
send MIDI Continuous Controllers (CC#11, 2, 3, 4 in order).
Arturia - MatrixBrute Manual - Operation 27