Reports the circuit ID in use at this level: on a point-to-point circuit with a fully initialized adjacency, it's the negotiated circuit ID from adjacency initialization; on a point-to-point circuit without such an adjacency, it's the concatenation of the local System ID and this circuit's isisCircLevelIDOctet (what would be proposed); on other circuit types it's a zero-length string. An admin reads this when correlating this circuit's identity as it would appear to, or has negotiated with, a peer router.