Auxiliary sends are used for two main purposes:
- Sending foldback signal to musicians.
- Sending signal to effects units such as digital reverb.
Auxiliary sends can be either 'pre-fade' or 'post-fade'.
Pre-fade means that the signal is taken from a point before the fader.
This is suitable for foldback since a foldback mix can be constructed that is completely independent of the positions of the faders.
Post-fade is suitable for reverb since when you fade the signal out, you generally want the reverb to fade out as well, and be in proportion at all other fader positions.