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; as a 'total' bin it never rolls over on its own the way the 15-minute and 24-hour bins do -- it only resets on an explicit clear or a card reset, so a high count here just reflects long uptime, not necessarily an active problem. 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.