This counts IGMPv3 TO_IN, change-to-include, reports, which a receiver sends when it switches from exclude-mode filtering to include-mode filtering for a group's source list, an active mode transition rather than an initial declaration. An admin watching for source-filtering mode changes mid-stream, say a client narrowing which sources it wants after initially joining broadly, would see this counter tick up during those transitions. A spike in TO_IN reports coinciding with a known change in client behavior or configuration would confirm receivers actively renegotiating their source filters rather than simply joining fresh.