If I understand the question correctly, this is where a person will normally use a matrix send. The LR will go to a set of matrix outputs instead of directly to the speakers. So LR might go to two matrix outputs, one for your subs and one for your main speakers. This will give you individual volume, EQ, and processing of the subs and main speakers, but overall volume can be controlled from the LR main fader.
So if you turn up the LR fader by 3db, both the subs and mains will increase a corresponding 3db. But if you just want to turn the subs up 3db, you would go to the sub matrix fader and turn it up by 3db. The main speakers would not be affected by this change.
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