Hi Debz, as Mervaka says, the Sel buttons correspond to what has been setup in the custom layer.
The Custom Layer is configured Channel 1-16 by default, but any combination of inputs or mixes can be assigned to this page as with the other Qu models.
Go to Setup > Control > Custom Layer to change channels shown here and use the rotary control to select the required channel for the available slot.
We have a tutorial video for Qu-Pac and Qu-Control on our YouTube Channel: https://www.youtube.com/watch?v=t0kCfYwjxxc
This might be useful!
Let us know if you have any more questions.