Unicast is a one-to-one transmission from one point in the network to another point.
FIX is an example of a unicast protocol which is delivered over TCP. However, some exchanges make market data available over multicast.
Multicast is one-to-many, meaning that the exchange sends out a single price update and this then distributed to many subscribers. An advantage of multicast for the exchange is that there is less opportunity for different recipients of the data to receive the data at different times (which could cause market participants to complain about unfairness). Sending data via multicast is also typically more bandwidth efficient than sending data unicast.