Step 6: Buses (subgroups), Main mix, VCA's
One very common use of a bus is to assign all the drum channels (floor tom, kick, hi-lo tom, hi-hat, snare, etc.) to one bus. This way, the engineer can change the volume of the drumset in the total mix without having to change each fader on each channel. Other uses include groups of singers (girls, boys; leads, harmony), instruments (main, backup; brass, woodwind, percussion), etc.
Just remember that changing the fader on a bus is NOT equivalent to changing the faders on all of its assigned channels. It simply changes the volume of that particular mix of channels relative to the entire mix. Channels can be assigned to multiple buses, and changing the volume of one bus with channel 6 in it will not change the volume of channel 6 in any other subgroups.
Each of the subgroups also has a button allowing assignment to the main mix. In most cases, you will want everything combined into the main mix so that you can control the total volume of everything with one setting. Sometimes, however, you may leave one subgroup by itself; independent of the main mix, for whatever reason may apply. Usually each subgroup will have a left and a right fader, allowing for individual adjustment of the left and right aspects of that mini-mix. The main mix will also probably be stereo. There will be two outputs on the back of the board for left and right mix.
However, some mixers have another button to assign the main mix and any and all subgroups to a "mono" fader. This is another output on the back that is a single bridged out between the right and left of the mix out. This allows you to have a huge amount of possibilities of setups depending on where you have channels and busses assigned and what you have plugged in to the back. The mono will "bridge" the left and right channels, causing all sounds in each to be played in both. It will also, however, leave alone any left/right settings set before it.
VCA's are confusingly similar to buses. However, it helps to remember that buses route, and VCAs are simply a control mechanism. Contrary to subgroups, a change in a VCA will be exactly like moving the faders of all its assigned channels. Thus, any post-fader outs will be affected: post-fader auxes, the level of that channel in any subgroups, etc. If post-fader auxes are used for other main speakers like subwoofers, than a VCA will allow change in those too; where a subgroup would not.
VCA's are found only in larger boards.