This counter tracks the number of USTAT (Unsolicited Status) PDUs transmitted on this port, per the description, sent when SSCOP detects a sequenced-data gap serious enough to report immediately rather than waiting for the next scheduled POLL/STAT exchange. Because USTAT is specifically for urgent, unrequested error reporting, its rate is a more direct measure of detected data loss on this port's receive path than the routine STAT counter, and a rising trend here indicates loss severe enough that SSCOP judged it couldn't wait for the normal polling cycle. This is a stronger link-quality red flag than an equivalent bump in regular STAT traffic.