There are three important things:

A - Defines whether you hear the "master" in the headphones or not.
B - Activate the cueing of a channel.
C - Determines what you listen to in the headphones and at what volume.
In particular, by turning the "C" knobs (the one at the top) you hear the pre-listening, the master or the mix between the two...
However, I suggest you read the manual: everything is explained very clearly....