rxjs Unicast vs Multicast

Consider this code:

const myObservable = new Observable<number>(subscriber => {
    const rdn = Math.floor(Math.random() * 200) + 1;
    subscriber.next(rdn);
});
myObservable
    .subscribe(a => console.log('Subscription A', a));
myObservable
    .subscribe(a => console.log('Subscription B', a));
    
// Result: 
// Subscription A 137
// Subscription B 8

Observable is unicast because each observer has its own instance of the data producer. On the other hand Observable is multicast if each observer receives notifications from the same producer, like this:

const rdn = Math.floor(Math.random() * 200) + 1;
const myObservable = new Observable<number>(subscriber => {
  subscriber.next(rdn);
});
myObservable
  .subscribe(a => console.log('Subscription A', a));
myObservable
  .subscribe(a => console.log('Subscription B', a));
 
// Result:
// Subscription A 149
// Subscription B 149

This last code snippet is not considering some treatments like errors and completions.

Subject is multicast because of this:

Internally to the Subject, subscribe does not invoke a new execution that delivers values. It simply registers the given Observer in a list of Observers, similarly to how addListener usually works in other libraries and languages.