This read-only Counter64 is the count of bytes dropped for this QoS Class on the VPath. An admin uses this to see whether a specific traffic priority level is experiencing loss, which matters far more for a realtime class than for a bulk class where retransmission is tolerable. For example, nonzero bytes dropped on the realtime class while the bulk class shows heavy drops is expected behavior (bulk is meant to absorb congestion), but drops on the realtime class itself would be a real concern.