This read-only PerfCounter64 counts Bytes Tail Dropped by this traffic port's QoS shaper during the interval, meaning bytes discarded because the port-level output queue was completely full. A rising count tells an administrator that this specific physical port's shaped egress buffer cannot drain fast enough for the traffic it is receiving. For example, if only one port on a shelf shows climbing cmTrafficPortQosShaperStatsBTD while its neighbors stay clean, that isolates the congestion to that single physical interface.