Exactly as described, this is a running total of multicast packets received on this benchmark port. Because standard benchmark test traffic doesn't use multicast addressing, a rising count here during a test indicates multicast control-plane or other unrelated traffic sharing this port, which an operator should exclude when interpreting the unicast-based frame-loss KPI so it isn't skewed by unrelated packets. For example, a burst of multicast packets coinciding with a spanning-tree topology change elsewhere on the network would show up here without representing any actual test-traffic loss.