This counts instances where the carrier identification field within the SIP3 header extension was invalid, one more granular check on the header extension alongside its length and version. An admin uses this specifically when troubleshooting multi-carrier SMDS interconnection issues, since the carrier field identifies which carrier network a PDU is associated with for inter-carrier billing and routing purposes. For example, PDUs arriving from an interconnected carrier's network with a garbled carrier-identification field would surface here, pointing toward an interoperability issue at that specific carrier boundary rather than a purely local problem.