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The 4 Channel User mode (found in Setup > Surface and Lights) allows you to change how the custom controls section of Fader bank 3 (Master section on the right of the console) operates. 

When in Bus Send mode, when you press F1/2/3, the 4 encoders are then used as sends to a bus. 
If you adjust this setting to Custom you can then assign custom controls to the encoders and Function buttons. 

Enable the Custom Mode an then press the View button in the Custom Controls section (Above the Jog Wheel). 
Press the F1/2/3 button and you can then assign parameters to the below encoders and buttons.