This is the highest value recorded during the current performance-monitoring interval of the residual bit error ratio measured after forward error correction has attempted to fix the errors detected on the incoming signal, expressed as a bit-ratio. This is the worst moment of the interval, and any non-zero maximum here means uncorrected bit errors reached the traffic layer during that period, which for many services means real data was corrupted. For example, if bitErrorRatePostFecMax spikes to a non-zero value during a single interval overnight, an admin would pull that interval's other counters, like pre-FEC BER and OSNR, to identify what impairment briefly overwhelmed FEC.