nbsCohpmRunningMaxPDL is read-only and holds a cumulative statistic since the port was last configured (i.e. since boot-up or the last protocol/FEC/preference change) for the maximum polarization dependent loss (PDL) on this coherent optical interface, i.e. how much the fiber path's attenuation varies depending on the signal's polarization state; not-supported is encoded as 0xFFFFFFFF. Unit measurement: 0.1 dB. Because it accumulates from the port's last configuration event rather than resetting every interval, a sysadmin uses it as a long-run baseline for this optic, and a value that drifts far from what nbsCohpmCurrentMaxPDL is showing right now can indicate a slow-developing fiber or transceiver issue.