Unsurprisingly, pressing the MOD MATRIX button opens the MODMAT (which is short for MODULATION MATRIX) page:
By default, the MOD MATRIX maps CC S#1 (STRUCTURE continuous controller slot number one) to MIDI CC # 1, STRUCTURE CC slot #2 to MIDI CC # 2, etc. The CC S# value is sent to the NODE parameters when used as modulation source.
Pressing the CHANGE soft key will open a window where you can change the incomming MIDI CC number that is mapped to the STRUCTURE CC slot. You can ‘MIDI Learn’ an incoming CC value by holding down the LISTEN MIDI soft key or manually setting the value with the ENCODER knob to choose the CC number.
Press and hold the QUICK CHANGE soft key and use the ENCODER knob to change the incomming MIDI CC number instantaneously.