Traditionally, you can use audio subgrouping to control multiple channels with one fader. However, many consoles -- especially digital consoles -- have VCAs alongside or in place of subgroups.
Let's look at how subgrouping and VCAs work, how they're different from each other, and how to use VCAs when constructing a live sound mix.
Subgroups offer the ability to sum several signals into one group, through a single additional amplification stage, and then routed via panning to the left and right master bus. This works great, except for two things: first, it adds one extra step of gain to the signals routed together, and two, it can cause issues when all the electrical signals are routed together into one circuit.
When a channel is assigned to a VCA, instead of summing the channels into a single subgroup, the VCA controls the fader voltage level without summing the actual signal to one singular circuit. Your signal remains pure, and you maintain your routing preferences on the channel without any additional gain or assignments necessary.
Traditionally, audio subgroups essentially work as a another send, with level tied to that of the master fader. However, they're a lot different than VCAs in many ways; one of the most notable is that most VCAs do not allow hardware inserts, much like subgroups; However, with VCAs, you have much more versatility -- you can mix mono or stereo tracks together, and on digital boards, apply helpful automation to your VCA groups.
When mixing on a VCA console, I'll usually make four VCA groups: one for drums (minus toms, in stereo), one for toms (because some songs require more presence in the drum kit), one for guitars, and one for vocals. I'll also sometimes have an extra VCA for auxiliary instruments or backing tracks.
This allows me to mix multiple channels easily without reaching across a large console, and also maintaining signal integrity across the whole signal chain.
That isn't to say subgroups aren't great, too -- in fact, my favorite trick on drums is to insert a high-quality stereo tube compressor across two subgroups (one for left, one for right; remember, subgroups are generally mono), bus all drums to those, and bring it up behind the original drum tracks. It adds body and punch where it's needed the most!
Mixing with VCAs -- or their little brother, the subgroup -- can be rewarding and translate into less stress and better mixes come showtime -- and using VCAs in the studio can mean a lot of control over an otherwise unruly mess of tracks.