This counter tracks the number of USTAT (Unsolicited Status) PDUs received on this port, per the description, meaning the far end has urgently reported a sequenced-data gap it detected without waiting for this port's next POLL. A rising count here reflects loss that the far end judged severe enough to flag immediately, so it's a more urgent link-quality signal than the routine STAT exchange and worth correlating with any physical-layer error counters for the link into this port. Frequent USTAT receipt from the same peer suggests a persistent, not intermittent, loss condition.