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 sits in the rolling window of completed 15-minute history bins 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, within that completed 15-minute window.