Sends on faders
Sends on faders allows you to quickly access all sends to a certain mix. When enabled all faders will show the level send to the selected bus instead of the LR mix.
This button enables/disables the sends on fader mode
In the mixer sidebar you can select the target mix. This mix will be shown when sends on fader is enabled. The current target mix is shown in the menu bar.
The bus master is the channel of the mix which is currently shown in the mixer. If sends on fader is disabled this will correspond to the LR mix. Otherwise to the selected target mix.