This is the increase in transmitted multicast packets since the last collection cycle. This rate is what an admin actually watches for multicast storms, which behave like broadcast storms but specifically implicate multicast-heavy protocols (e.g. a misbehaving clustering heartbeat) rather than general broadcast noise. A delta that suddenly jumps by orders of magnitude on this interface, distinct from the broadcast delta staying flat, would specifically implicate a multicast-based application misbehaving, not a general Layer 2 storm.