This is the peak (maximum) Pre-FEC Bit Error Rate observed during the bin at the OTU layer, encoded as a PmCounter64Atto atto-scaled integer, per the MIB's description. Because the average pre-FEC BER can hide a brief spike, this max value on this specific bin is what catches a short burst of degradation -- e.g. a micro-bend flexing in the wind or a transient laser wobble -- that an averaged reading for the same interval would smooth away; 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. If the average pre-FEC BER for this bin looks healthy but this max field corresponds to something like 1e-9, that gap alone tells an admin the link had at least one rough moment worth investigating within that window.