This read-only PerfCounter64 counts Frames Tail Dropped by this network port's QoS shaper during the interval, meaning frames lost because the network-facing port's shaped queue was completely full. An administrator uses it as frame-count evidence of congestion on the aggregation uplink itself rather than on any single customer-facing port. For example, seeing multiple customer complaints at once alongside a spike in f3NetPortQosShaperStatsFTD would point to shared uplink congestion rather than an individual port fault.