It seems that PMUX setting B ("analog functionality") of SAMDxx (I'm looking at SAMD51, specifically) has up to 7 different functions associated with it, and some pins can do 5 of them:
PA02 ADC0 AIN0 pmuxB
PA03 ADC0 AIN1 pmuxB
PA03 ADC0 X0 pmuxB
PA03 ADC0 Y0 pmuxB
PA03 ADC0 VREFA pmuxB
What controls which one is "active" ?