"Topic" is featured prominently in the chapter title, but it's not discussed much at all in the enclosed text. It shows up in the Publish-Subscribe section, but the link to topics is mostly implicit.

One of the key things you need to know is the default routing type. It's a bit hard to say if it applies to all cases, but the closest thing is the text in the advisory note:

"""
If an address does use both routing types, however, and the client does not show a preference for either one, the broker typically defaults to the anycast routing type.
"""

That "typically defaults" is in my view unnecessarily squishy. And is this in general correct? Or is it making a narrow point when we have not yet clarified the general case: the default routing type?