Apple Logic Pro 9 - Manual (Page 1117)

The GM mixer is intended for controlling the 16 channels of a MIDI device which conforms
to the GM, XG, or GS standard. This includes the GM set of 128 program names (with
variation banks for XG or GS), MSB/LSB bank select messages, and standard controller
names (#1 for mod wheel, #2 for breath, #7 for volume, #10 for pan, #11 for expression,
and so on).
The Parameter Box of the GM Mixer
Most of the GM mixer’s setup is done in the mixer itself, but there are also a few parameters
in its Parameter box.
Port and Channel
Like the instrument and multi-instrument objects, the GM mixer can also have a direct
MIDI output connection. It is set with the Port parameter.
The Channel parameter determines the channel number of the lowest GM mixer module.
You can use this (coupled with resizing the GM mixer) to create sub-mixers for any
continuous string of MIDI channels (1 to 4, for example).
Legend
The Legend parameter turns the display of the legend along the left edge of the GM mixer
off and on.
The legend indicates what the rows of controls do, and is also used to set the function
of the top four rows of knobs. After the GM mixer is set up, you can hide the legend to
save space.
1117 Chapter 37 Environment Objects Reference