nbsCoherentStatsMaxQ is read-only and holds the maximum Q factor observed on this coherent optical interface since statistics collection last started or was cleared (nbsCoherentStatsState), i.e. a composite signal-quality figure of merit derived from the coherent receiver's error statistics, where a higher value means a cleaner, lower-error signal; not-supported is encoded as 0xFFFFFFFF. Unit measurement: 0.1 dB. A sysadmin treats it as a longer-run baseline for this specific optic/fiber path, useful for spotting a slow drift in link quality that a single quarterHour or twentyfourHour sample in nbsCohpmCurrentTable might not reveal.