This read-write value identifies the SSI (Subscriber-Network Interface, or a similar switch-specific signaling/service interface) to which this SMDS address is associated, a further layer of association beyond the plain interface index (smdsaddrIf) and slot (smdsaddrSlot). This ties the address to whichever specific SSI logical construct the switch uses to manage subscriber-facing SMDS service delivery. An engineer diagnosing why an SMDS address isn't receiving expected SSI-level service treatment would check that smdsaddrSsiIfNum correctly points to the SSI instance configured for that customer's service profile.