This field reports the number of packet drop events on this PM instance, exposed here as cienaCesPmExtendedTxRxAllTimeBinDropEvents. On this Ciena CES port's PM data, this is an RMON-style drop-event counter — it counts the number of times this PM instance had to drop packets due to resource exhaustion (a full buffer or queue), not the number of packets dropped, so one burst of congestion that drops fifty packets in a row can still just register as a single event here; and this 'all-time' counter for the extended Tx/Rx PM group has no fixed rollover schedule of its own — it just keeps accumulating from whenever the PM instance was last cleared, so a single reading only tells you the running total, not a rate. For example, drop events climbing from 0 to just 1 on this port would already be worth a closer look, since a healthy link should hold this counter at zero.