Multicasts are single data streams read by any number of clients with the appropriate decoder. Multicasting is a unidirectional communication. Clients simply listen to a data stream. A client that connects in the middle of a multicast misses half of the data stream. Unicasting is a bidirectional communication.
Clients send requests to the server, and each connection from client to server uses bandwidth.
Most of the NIH network is multicast enabled. Live events streamed to computers on the NIHnet are mostly multicast, thus saving considerable bandwidth. However, the same live event streamed to computers on the Internet or to non-multicast enabled segments of the NIH network are unicast. On-demand viewing of past events, whether on the NIH network or the Internet are always unicast.