This is the Post-FEC Bit Error Rate Estimate at the OTU layer, encoded as a PmCounter64Atto atto-scaled integer -- a model-derived estimate of the residual error rate remaining after FEC correction is applied, per the MIB's description. Post-FEC BER on this section is essentially the 'did FEC actually clean it up' number -- it should sit vanishingly close to zero on a healthy link, and any measurable uptick here (even while pre-FEC BER looks only moderately worse) means FEC's correction headroom on this specific port is starting to run out; this particular bin is a live counter that keeps accumulating until it is explicitly cleared rather than rolling over on a fixed 15-minute or 24-hour schedule, so a large value mostly reflects how long it has been since the last clear. A healthy reading for this bin would correspond to well under 1e-15, essentially error-free after correction, whereas a jump toward 1e-9 would be treated as a leading indicator that this OTU section is approaching its FEC correction limit.