This reports the Post-FEC Bit Error Rate Estimate on this OTU signal -- the residual error rate the receiver estimates remains after FEC correction has been applied. Because this reading comes from a closed, completed 24-hour history bin for this OTU signal, this is the number that most directly predicts whether client traffic on this OTU signal will actually see errors, since it is what is left over after the correction engine has done its best. For example, a Post-FEC BER estimate of 1e-15 on this OTU signal means the corrected output is effectively error-free for client traffic, while a rise toward 1e-9 would start to be noticeable as occasional packet loss upstream, across that completed 24-hour day.