This is the number of times a PDU is received with a different value for ID field length than that of the receiving system. IS-IS requires every router in an area to agree on the System ID field length, and this counter increments whenever this router receives a PDU declaring a different ID length than its own, typically pointing to a misconfigured or non-standard neighbor rather than to this router itself. For example, a legacy or third-party device configured with a nonstandard System ID length joining a segment where all other routers use the standard 6-byte ID would cause this counter to climb on every properly configured neighbor receiving its PDUs.