The twelve Central User Buttons can be used for master functions such as enabling the second metering row on the Channel Displays; start/stop/reset for integrated loudness measurements; copy and paste for copying levels to auxes, etc.
All user buttons are programmed from the Custom Functions display and are stored at a system level. This means that any changes will affect all users.
Editing the Functions
There are no default functions and so the user buttons must be programmed from the Custom Functions display. From there you can define the button label, color and function.