The output section has faders for the 8 group outputs (buses) and the Left/Right (L/R) MIx (on the right). Any of the buses can be soloed.
The Mackie 8-bus board has the ability to use the 8 group outputs as subgroups for the Left/Right mix. For example, if you were designing a stereo mix but wanted a group fader to control all your synths and another group fader to control all your mics, you would follow this procedure:
1) unassign your mic and synth channels from the L/R Mix
2) select a group you would like to use as a subgroup for the synths and assign the synth channel strips to that group: do the same for the mics (for example, assign the mics channels to group 1-2 and the synths to group 3-4)
3) pan each channel strip where you would like to image the instrument or mic in the subgroup (unless using MONO)
4) above the subgroup fader, choose L MIX, R MIX, or L+R MIX, depending on where you want your subgroup imaged. Normally, to keep the stereo image, you would have the odd-numbered groups assigned to L MIX and the even-numbered groups assigned to R MIX
5) if you really want a mono subgroup, be certain to depress both L and R MIX in addition to pressing the MONO button above
6) these faders will now act as subfaders to the Left/Right MIX fader, so you can fade out entire groups with a single fader, rather than grabbing all the individual channels faders