It is one of the aux sends. You set the master aux in the center section to noon or so and slowly bring the same aux up on the channel strip. Adjust as needed.
MTX is matrix, I would not think that would be used as a monitor, but it could. Aux are the Grey, Blue and Yellow knobs. Also check the little blue recessed switch in the master section. If I recall correctly that switched the master aux knobs with the sub group below.
