This counts frames that the network attempted to deliver on this circuit but which never arrived -- actual loss, as distinct from cktInDiscards, which is intentional rate-limiting -- making this the counter an admin checks first when a customer reports dropped data or session timeouts on a circuit that isn't obviously oversubscribed. For example, a circuit with normal cktInDiscards, implying the customer isn't oversending, but a nonzero and climbing cktOutLostFrames points to a real network problem -- congestion or a failing trunk along the path -- rather than the customer simply exceeding their CIR.