This counter records the number of bytes dropped from the tail of the access port's shaper queue during the archived historical interval. A nonzero historical value confirms to the admin that buffer overflow congestion actually occurred on that port during that past period, useful evidence for a retroactive complaint investigation. For example, if a subscriber's ticket references buffering last night, the admin would check cmAccPortQosShaperHistoryBTD for that interval to confirm tail drops occurred.