VNX DAE compatibility

I'm curious if there is any difference whatsoever between the "N" and "R" lines of VNX disk array enclosures.  They come in 15 or 25 slot, and some made specially for the VNXe series which end in E, but I can't find any documentation suggesting they're otherwise any different.  Can anyone shed some light on this?

Example part numbers:

V31-DAE-N-15 vs V31-DAE-R-15

V31-DAE-N-25 vs V31-DAE-R-25

V2-DAE-N-25-A vs V2-DAE-R-25-A

V31-DAE-N-15E vs V31-DAE-R-15E

V31-DAE-N-25E vs V31-DAE-R-25E

You get the idea...

Thanks!