This read-only Counter64 reports the number of packets the MMU discarded because they aged out of a queue before they could be transmitted, counted across the whole switch's forwarding ASIC, aggregated over every port (the per-interface breakdown of these same drop reasons is in the parallel dellNetFpIf... table). This is a Broadcom-style switch-ASIC hardware drop counter: an admin troubleshooting unexplained packet loss walks this whole family of counters (Ingress*, Eg*, MMU*) looking for whichever one is actually incrementing, since that pinpoints the specific stage of the forwarding pipeline (STP state, ACL policy, buffer exhaustion, CRC error, TTL, etc.) where packets are being discarded, rather than just seeing generic interface drops.