PPMU by most ATE manufacturer definition is Per Pin Parametric Measurement Unit. By this it means each digital channel pin has its own Parametric Measurement Unit.
For the PXIe-6571, it is quite misleading to say it has PPMU but actually each bank of 8 digital channels has a shared Parametric Measurement Unit.
It should be renamed SPMU (Shared Parametric Measurement Unit).
Per-Pin Parametric Measurement is kind of still valid because all 32 pins are capable of sourcing/measuring. Just that the measurement circuit is multiplexed internally, in other manufacturers, they don't typically tell if the measurement circuit is dedicated per pin or shared by multiple pins.
In any case, the functionality remains the same, the only difference you would observe is when taking measurements. For example, with an aperture time of 1ms, if you were to measure on ch0-7, it will take 8ms in total as all these channels share the same measurement circuit. Whereas, if you were to measure ch0,ch8 it will take just 1ms as those channels have separate measurement circuits.
In simple terms, the worst-case measurement time is 8 x aperture time.