Unknown multicast frames are frames destined to addresses not registered in IGMP snooping.
This product forwards unknown multicast frames to all ports as the default setting, which does not matter in low bandwidth environments. However, discarding instead of forwarding in high bandwidth environments may be recommended.
If you want to discard unknown multicast frames and only forward some multicast frames that use link local addresses such as mDNS, you can exclude them from discarding.