the PRC stands for DeCarbon type with Rebound and Compression, I suspect the PRXB has the high and low speed Compession damping, as I think thats the coding for the unit on my R6
As skidlids rightly says, the prxb has high and low speed compression adjustment. Also it has the dial-type preload adjustment where the prc has the c-spanner rings. That's what I understand from ohlins website anyway!