This PmInteger32Milli field reports the received optical power on this PTP-carrying optical channel, in milli-units (thousandths of a dBm), for the most recently completed PTP history bin. Because this row is a closed, completed interval from the PTP performance-monitoring history table for this timing session, tracking received optical power alongside PTP timing counters lets you tell whether a degrading optical link (a dirtying connector, a failing transceiver) is the underlying cause of timing errors elsewhere in this same PM row, rather than a problem in the PTP protocol stack itself. For example, a reading of -8500 (i.e. -8.5 dBm) that has been drifting downward bin over bin toward a receiver's sensitivity floor would explain why PTP timing accuracy on this channel has started to degrade, and would point at the optics rather than the clock.