"DEFINEs in PBP are converted literally to Assembly Language #DEFINE directives." from the PBP manual.

The "DEFINE USB_RXrcv POTRD,2" in PBP is converted to "#DEFINE USB_RXrcv PORTD,2"
An Assembly "#Define" in this case is just a text substitution.
In the Assembly code, wherever "USB_RXrcv" is, it is substituted (replaced) with "PORTD,2"