We call RxJs Subject as multicast because it shares the same observable (data producer) to all its subscribers meaning it shares the data with all its subscribers

where as Observables has two parts data producer (Observable) and data receiver (subscriber). In Observable, Each subscriber owns independent execution of observable so it will not share the same value to all its subscribers.

Unicast : Each subscription owns independent execution of observable, it passes notification to a single subscription. Multicast : It shares the execution of Observable and it passes notification to all it's subscribers.