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Hi TJ, the difference is simply that the ‘M’ in LR+M is a separately switched mono mix bus, while the ‘M’ in LR+Msum is a mono sum of the LR mix. In your case the LR+M would be better as you could switch selected channels to feed the M output for the sub.

Press Mix on the main LR master strip and use the Assign key to assign channels to the LR stereo bus. Press Mix on the main M master strip to assign channels to the M bus. Channels can be assigned to both the LR and M buses at the same time. You would want to do that with your sub feed. For example, the kick drum would be assigned to both LR (tops) and M (sub), but a vocal would be assigned to just the LR bus (tops only).

I hope that helps,