The suggestions of “Your topic is similar to …” when posting and the “related topics” which pops up occasionally so far have not been useful. In fact they have been generally ridiculous.
I’ve not found the suggested topics at all useful either, myself, beyond seeing the unread/new items.
My guess is that this will improve with time as the number of posts increases and similarities start showing up.
Right now the new topic compose suggestions are based exclusively on the title of the topic which is not optimal. We do hope to improve this over time.
(I’m not sure it even takes category into account, but it definitely does not look at the body yet.)