They’re closely related but this is regarding a category I’m tracking, and should therefore be tracking all topics within it unless ever I opted otherwise on a per-topic basis.
So here I’m talking about topics that I think should be showing up as Unread. (I.e. it appears the system is not working exactly as intended)
The other topic is regarding topics which are not in tracked categories, but should still show up as 
when certain conditions are met. (I.e. a new feature.)
It has indeed gotten confusing though, because I believe this:
100% solves this: Completely unread topics with recent replies should show as new
But it is only a partial solution to the Unread problem I’m raising here. When a category is tracked, old topics with new replies should show up in Unread. But maybe we can live with this little logical discrepancy. The important thing is that topics are not missed and email-only users receive an update.