This read-only Counter64 counts the cumulative number of bytes discarded by tail-drop for this row's traffic class. An admin uses this to distinguish simple queue-full tail-drop losses from congestion-avoidance drops like WRED, helping pinpoint whether a class's queue depth is simply too shallow for its traffic. For example, a steadily climbing zxr10QosStatBytesTailDrop count on a "data" class suggests its queue is regularly filling completely and needs a larger buffer allocation.