A bus is a virtual path that carries an audio signal to another place inside a DAW or a mixer. You use it to gather several tracks into the same processing destination, or to deliver a sent signal to an aux. Think of it as patching real cables inside the software.
Making a submix
Assign the outputs of several drum tracks to the same bus, and process the whole group on the aux that receives it. You can ride one fader or compress the entire kit at once while keeping the internal balance.
Using it as a send destination
If you point the sends of several tracks at the same bus, the branched signals gather into a single aux. When you share a reverb or a delay, the bus connects the source tracks and the effect return.
How it differs from an aux
A bus is the name of the signal path, and an aux is the channel that receives the signal coming from a bus. Terms and displays differ by DAW, though, and the same role may be built with names such as group, return, or FX channel.
Keep it organized
Bus numbers alone make the purpose hard to read, so give them names that show their role, such as Drums or Vocal Reverb. Handy tips for organizing tracks in Logic Pro (Japanese) also covers the difference between summing stacks and buses.
Routing cautions
If the source and the return loop back on each other, feedback occurs, so check the flow of the signal. When a sound seems doubled in level, look for an unintended overlap between the output of the original track and the path through the bus.
Think in roles, not numbers
Bus 1 and Bus 2 are identifiers for the routing, and there is no difference in sound quality or function. What matters is what is being sent where, not which number you use. If you change what a bus is for, review its name and the source settings at the same time.
