+facebook_is_better_than_bonfire boundaries and circles are integrated in topics, so you were able to add this sub-topic because the parent topic has open boundaries (public), but you can create topics where only some circles can publish (in which case things they tag end up in the Submitted tab and don't appear in the feeds of topic followers) or create sub-topics, etc
they share similarities, but the focus of topics is more about collecting and curating information to share with a broader scope (eg. all subscribed users) - think of it more as a "magazine" with its editorial team, or "channels" (like in several chat applications) but on steroids
Topics are similar to the way a.gup.pe implemented groups on the fediverse. I think of them more like "federated hashtags".

You can include one or multiple topics (using + mentions, which share similarities with both @ mentions or # tagging) is the text of a post, and that post is still published in your timeline and your followers' feeds as usual, and in addition submitted (or published directly, depending on the topic's boundaries) to the topic's timeline and to followers of the topic.

This is different from typical groups where you usually post 'in' just one group at a time, and the group moderators sort-of "own" the post (they can delete it for example, whereas with topics they only control whether the post is shown/shared by the topic). This also does away with cross-posting something in several places, since one canonical post can be shared in multiple places.

In our implementation, you can also add tag something you or somebody else posted (depending on the post's boundaries) at any any time, not only when initially posting.