This PerfCounter64 counter reports a count of frames dropped because of an ACL rule on the flow. An admin watches it because, unlike a generic error counter, this specific drop reason tells them which mechanism on the flow 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 cmFlowStatsACLA2NDrop on flow 29 shows 303 hits for the current interval while every other drop counter on the same flow stays at zero.