This counts frames received on this circuit that carry the FECN bit set, meaning some node upstream toward the source experienced congestion while forwarding this traffic toward this circuit's destination. A climbing cktInFECNFrames count tells the admin that traffic destined to this circuit's far end is hitting congestion somewhere in the path, information a receiving application could in principle use to throttle back. For example, a sustained increase in this counter during business hours would corroborate a customer's complaint about slow response times by showing the network itself flagging congestion on frames meant for them.