RS = Reed Solomon - An algorithm for inserting error correction data prior to transmisssion to assist correction when received. It's a type of Forward Error Correction (FEC)
http://en.wikipedia.org/wiki/Reed%E2...ror_correction
The pre and post error counters should show how well the error correction is performing i.e. Pre-RS may be non-zero but Post-RS should be very low, ideally zero.
If the values reported by the SH are incorrect then the counters are, of course, useless!