Several names for same/similar registers in PIC32MZ USB peripheral?
It's probably too late to be of any use but all the USBIENxxxx registers are related to whichever endpoint is pointed to by USBCSR3bits.ENDPOINT. So if you set ENDPOINT to 1 and then access USBIENCSR0bits you're actually accessing USBE1CSR0. The reason they may look different is if ENDPOINT is 0 it points to endpoint 0 and the registers for endpoint 0 look different from all the other endpoints.
It's very confusing and I don't know why they did it like this. The documentation is pretty poor!