This counter tracks the number of SD (Sequenced Data) PDUs transmitted on this port, per the description, where SD is the SSCOP PDU type that actually carries the Q.93b signaling messages (SETUP, RELEASE, STATUS, etc.) as payload. This is effectively the aggregate signaling throughput of the port at the SSCOP layer, so it should scale directly with the sum of all the individual Q.93b message counters like svcAtmConfigQ93bNumSetupPduTx, and a mismatch between the two would indicate accounting or layering issues worth double-checking. It's the single best 'how busy is signaling on this port' number available at the SSCOP layer.