This counts frames the switch delivers on this circuit with the FECN bit set, meaning the switch itself is notifying this circuit's receiving end that congestion was encountered somewhere along the path the frame traveled to get here. A rising count here, delivered to a customer's router capable of acting on FECN, should correspond with that router adjusting its own traffic shaping; if it doesn't change behavior, the notification is effectively being ignored. For example, an engineer investigating chronic slowness on a circuit might check whether cktOutFECNFrames is elevated to confirm congestion notifications are indeed being generated and delivered, even if the customer's equipment isn't responding to them.