Ahh… Thank you for the report!
Uhh… I think “linked” here is a bit too vague, so I’ll try to answer as best as I can. You can add multiple topics (or any URL) to a subcollection’s list. Similarly multiple subcollections can list the same topic or URL. Subcollections are very simple, and are effectively just a list of URLs. The only restriction is that only one topic can display a single subcollection. This last restriction of one topic displays one subcollection won’t change.
Ahh, that field is supposed to be URL only for subcollections. I originally intended subcollections to mainly serve posts, but searching for that would be painful. I think I’ll have to update the table header here to avoid confusion.