This narrows cktOutLostFrames down to specifically the discard-eligible portion of lost traffic, telling the admin how much of the total loss on this circuit was 'expected' loss of low-priority frames, which is normal and by design under congestion, versus loss of higher-priority committed traffic. For example, if cktOutLostFrames and cktOutLostDEFrames are nearly equal, essentially all the loss on this circuit is coming from its lowest-priority traffic exactly as the discard-eligible mechanism is designed to behave, which is a healthy sign rather than a fault.