Exactly as described, this is a running total of multicast packets transmitted by this benchmark port. Because ordinary RFC2544 or Y1564-style benchmark test traffic is unicast, this counter incrementing during a test is unusual and typically indicates something other than the test itself, such as multicast control-plane traffic sharing the test port, rather than the benchmark payload contaminating results. For example, a nonzero and growing value here during a supposedly isolated point-to-point test would prompt checking whether the port is also carrying unrelated multicast protocol traffic.