Specifies the method used to calculate variable number NN from MIDI CCs. Mult is multiplication, with 1 being full scale - 0 at a CC value of 0, and 1 at CC value of 127 - this is how the amplitude opcode works. Add is addition - how volume, cutoff and all other modulation destinations work by default when modulated.
Here is how multiplication could be used, together with varNN_target (in this case, var01_cutoff as 01 is the variable number and cutoff is the target) and varNN_onccX to control velocity tracking on a filter cutoff:
//Lowpass filter cutoff=120 cutoff_cc102=8400 fil_keytrack=100 resonance=0 resonance_cc103=24 var01_cutoff=4800 //Velocity track var01_mod=mult var01_oncc131=1 var01_oncc111=1