Auxiliary Send Masters   

These adjustments act globally on any signals which are being sent to the auxiliary bus.
This provides an easy way to adjust these levels while maintaining the relative send levels set on each input channel. 
For example
If several signals are being routed to send number 1 and the resulting send output is too high for the destination device distortion may result. You can make adjustments to each of the individual sends at each input channel. 
A quicker solution is to lower the Aux Send Master 1 which will reduce all those signals and provide a quick remedy.