I don't know exactly the layout of your mixer, but the submix faders can be used for a few things. Lets say you have a drum kit coming in on inputs 1 to 4 of your mixer, from your multitrack device. You could assign all these inputs to one of the submix faders on your board, and control the level of all four with one fader. This is handy if you want to dynamically mixdown the song, bring the whole drum kit down a bit here, up a bit there etc..
Another way you can use these (again, depending on how your board routes signals) would be creating a customized headphone mix for 2 of the musicians recording, bass player needs more drums, drummer needs more of everything etc..
Usually the submix faders have a way of routing to the main L and R outputs for final mixdown. Or, for doing a headphone monitor thing, take the outputs from the submix groups into your headphone monitor amps (seperate mini-amplifier for headphones only).
Emeric
