Number: 80, 81, 82, 83
Whatever the musician sets this controller to affect. There are 4 General Purpose Buttons, with the above controller numbers. This are either on or off, so they are often used to implement on/off functions, such as a Metronome on/off switch on a sequencer. If a MultiTimbral device, then each Part usually has its own responses to the 4 General Purpose Buttons.
0 (to 63) is off. 127 (to 64) is on.