This counts IGMPv3 IS_IN type membership reports received, the report type a receiver sends to declare it wants to receive traffic only from a specific, listed set of sources for a group. An admin verifying source-specific multicast (SSM) adoption on the network would watch this counter grow as clients properly declare included-source interest, confirming SSM behavior is actually happening rather than clients defaulting to broader any-source multicast reports. A healthy, steadily incrementing count here on an SSM-focused service confirms clients are using the expected precise source-inclusion signaling.