You can connect a momentary foot pedal—with either positive or negative polarity—to the foot switch sockets. By default, foot switches control the following functions:
USER SWITCH A is assigned to Start/Stop.
USER SWITCH B is assigned to Record. (Note that a track must be selected and armed for recording to take place.)
EXTERNAL CONTROL is assigned to the MASTER fader level. Only use an expression pedal with this socket.
The polarity of the foot switches is determined by the Mackie Control when powered up. You should first connect the foot switches, then turn on the Mackie Control.