This PerfCounter64 counts Bytes Tail Dropped by a QoS Shaper in the history bin, i.e. bytes dropped because the shaper's queue was completely full when they arrived. An admin distinguishes tail drops from RED drops in history to tell whether a shaper's queue is regularly overflowing outright rather than just triggering early congestion avoidance. If f3FpQosShaperHistoryBTD is consistently nonzero across many bins for a given customer's shaper, the admin would consider increasing that shaper's queue depth or committed rate rather than relying on RED tuning alone.