The job of the Gain/Trim knob is to control those levels so the faders can best control the volume output.
Gain staging is important because it directly pertains to the volume relationships within our mix.
How do you set a gain structure?
To establish proper gain structure, the primary element and first concern is input gain. Each system input provides adjustable Gain In or trim level, with an associated Peak indicator. For best performance, increase the gain on a given input until the Peak indicator just begins to flash on normal signal content
Rule of Thumb
1- follow the signal
2- Strong signal in sound check
3- Set the fader with the gain