This is the number of IS-IS CSNP frames seen in this direction at this level. Since only the DIS periodically sends CSNPs on a broadcast circuit, while point-to-point circuits use them only once right after the adjacency comes up, an admin seeing a steady periodic stream of CSNPs on a given circuit can infer it is a LAN segment where this router either is the DIS (if transmitting) or is not (if only receiving). For example, a point-to-point circuit's CSNP transmit counter incrementing only once right after link-up and then staying flat is the expected, healthy pattern, whereas continued periodic increments there would suggest that circuit is unexpectedly behaving like a broadcast circuit.