This PerfCounter64 counter reports a count of jabber conditions detected (frames longer than the maximum legal size combined with a bad CRC), in the downstream (network-to-subscriber) direction. An admin watches it because, unlike a generic error counter, this specific drop reason tells them which mechanism on the network (uplink) 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 cmEthernetNetPortStatsESJ on network (uplink) port 27 shows 6 hits for the current interval while every other drop counter on the same port stays at zero.