With AUX AUXiliary output, aux auxiliary output can cut off other signals more conveniently. For example, if you cut off accompaniment and short films, you can only use microphones and buses to output them in groups. Of course, there must be enough groups to record. M can't do it, because the output of M is behind L~R, and LR must be all the sounds in the scene, so M can't just leave the microphone signal.