Meaning of the different ports in the XMEGA PR.PRPA and PR.PRPB power reduction registers?
This single line at the very end of the ADC section of the ATXMEGA64B3 data sheet references the ADCB name for the ADC...


  PORTB has one ADC. Notation of this peripheral is ADCB


There is no corresponding line for an ADCA (although this peripheral is referenced several other places in the datasheet). Note that the single ADC on this device is also called just ADC in the header files. 

I've empirically tested and the PPR bits and, in fact, the PPRB bit does disable the single ADC on this part. The PPRA bit, while defined in the headers, is not actually implemented on this device so setting it has no effect.