This counter records the number of bytes dropped from the tail of the shaper's queue during the archived interval. A nonzero value in a past interval tells an admin that congestion severe enough to overflow the buffer occurred at that time, evidence useful for retroactively explaining a customer complaint. For instance, if a subscriber reported buffering during last night's peak, the admin would check cmQosShaperHistoryBTD for that interval to see whether tail drops actually occurred.