A Source is something that can be converted to an Observable. The counterparty to Source is a Sink, something that can be converted to an Observer.

A Source could be an Observable itself, an EventBus, a Var, or, via implicits, a third party type like Future or ZIO.

The point of using Source instead of Observable in your API is to let the end users pass simply eventBus instead of eventBus.events to a method that requires Source, and to achieve that without having an implicit conversion from EventBus to Observable, because then you'd also want an implicit conversion from EventBus to Observer, and those two would be incompatible (e.g. both Observable and Observer have a filter method).