This PerfCounter64 counter reports a count of undersize packets received, shorter than the legal minimum frame size, in the upstream (subscriber-to-network) direction. An admin watches it because, unlike a generic error counter, this specific drop reason tells them which mechanism on the access port is rejecting traffic, which narrows troubleshooting to that mechanism instead of the whole data path. A subscriber's support ticket about intermittent loss becomes a five-minute diagnosis, not a lengthy trace, once cmEthernetAccPortHistoryESUP on access port 2 shows 337 hits for the most recent completed bin while every other drop counter on the same port stays at zero.