Not totally understanding your use case and question. But, as far as I know, you can’t “unassign” anything. You also can’t “mute” the channel as that mutes it to everything. What you can do is turn the fader to the Main L/R output all the way down, while leaving it up for other outputs. In the fader screen, select the “sends to” dropdown and select which outputs you want and you can mix differently to each output. The one problem with this, which is a bit of an oddity, is if you turn down the fader to the Main L/R, you will lose all FX going to the other outputs. Or simply don’t connect anything to Main if you’re not using it and/or you can mute the Main L/R if you’re not using it for anything else.
There are some routing quirks in this system that are a bit head scratching. But they don’t get in the way of most use cases as long as you understand them.
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