This TruthValue indicates whether there is currently a member of this specific multicast group present on this specific interface, per its description, and the object is read-create so an admin can also manually assert membership for a row. It's the group-and-interface-specific counterpart to the interface-wide igmpCondFilterIfRouter check, feeding Conditional Filtering's decision to forward a particular group's traffic onto a particular interface. Manually setting this to true when creating a row forces traffic for that multicast group to be forwarded to that interface regardless of whether IGMP reports are actually being received.