The following are usually assigned to on/off buttons on a MIDI keyboard:
MAPPING A CONTROLLER FOR SAKURA SYNTH MANUAL
MAPPING A CONTROLLER FOR SAKURA SYNTH MANUALS
This assumes two keyboard manuals and a set of bass pedals. A jazz Hammond organ player would want to be able to control each of these parameters in live performance. Here is a screen shot of the basic layout of the Vintage B3 clone in Apple GarageBand and MainStage.