Topics are simple enough as a concept; they wrap up what is internally arbitrary code emitting and matching on events. We don’t particularly have the set of concrete topics to expect yet, but this is how a general subsystem looks.
A topic is another available actor for the system; its job is to (internally) subscribe to events, so that it can form the correct topic-events, and then pass them on. This might require a bit of internal data rearrangement, but nothing more complicated than renaming fields into new data structure items.