This is the number of unknown IS-IS PDU frames seen at this level. This counts PDUs this router received that it could not classify as any recognized IS-IS PDU type, so a nonzero and climbing value is a specific signal of either a version or vendor incompatibility with whatever device is sending them, or a malformed PDU arriving on that circuit. For example, this counter starting to climb right after a new, non-Cisco IS-IS implementation was connected to that circuit would point the admin toward interoperability testing with that specific device as the next troubleshooting step.