Enable? Disable? but what?

WAN1 <-> LAN traffic

WAN2 <-> LAN traffic

WAN1 <-> WAN2 traffic?

Say, it is set DISABLED, what is / isn't blocked?

How clear or vague is ONLINE HELP? It reads:

Multicast Pass Through
IP Multicasting occurs when a single data transmission is sent to multiple  recipients at the same time. Using this feature, the Router allows IP multicast  packets to be forwarded to the appropriate computers.