Multicast is the case where one or more messages sent in a network is routed to a generalized subset of the systems in the network, including the null set, all of them, or any level in between. Unicast is the special case of that in which it is sent to exactly one of the systems, at least on the ultimate hop. Broadcast is the special case in which it is sent to all of them. Clearly there are algorithmic differences here. When I send to all the devices in a network, it is possible to use (as Novell Netware did at one time) an algorithm in which a router only accepts transmissions that succeed in an URPF test (we only accept messages from the interface[s] that we would use to forward a message to the source address of the message), and forwards them to all of its other interfaces. In unicast routing, there is a pretty specific network map generated. But in multicast routing, where one might send to any possible subset of the systems, one has to have some variation on joining and leaving the group and figuring out what subset of the links a message should be repeated on at each router.