This reports the live count of active interface-group membership pairs across the device right now, the usage counterpart to the group-member-interface ceiling. An admin during a large-scale IPTV rollout would watch this value's growth as more subscribers join more channels, since it's often the fastest-growing multicast resource in a deployment with many receivers each watching several channels. A rapidly rising value relative to the configured maximum, more so than the plain group count, could be the first resource to actually run out during peak viewing hours.