This counter tracks the number of UD (Unnumbered Data) PDUs received on this port, per the description, representing unassured (unacknowledged, unsequenced) data delivered to this port's SSCOP layer. Since this delivery mode carries no guarantee and no acknowledgement, this counter is informational rather than a health signal the way SD or ER counters are, and it should stay proportionally small relative to normal sequenced signaling traffic on a call-signaling port. A sudden large jump here would be more indicative of an unusual peer behavior than of a local fault.