Hi,
But what means (bit6-2) "11111 = FVR (Fixed Voltage Reference) Buffer 1 Output?[
It simply selects the output of the FVR and "connects it" to the ADC input in order to sample it, just as any other input.

What is this for?
It can be used to "calibrate" the ADC when you're not actually using the FVR as VRef for the ADC.

Must i use this for my reference voltage?
No, in your case, with the output of the FVR used as VRef for the ADC there's no need to sample the FVR output - it would/should Always return a result of 1023.

/Henrik.