Unread based on your personal notification preferences.
If you have “watching” set on all categories, then you will get notified about every new topic and every new post in that topic.
You can still browse the entire site — through Latest, through going to a category, through browsing a tag — and your read and unread status is tracked.
I wrote about it here:
And here’s another screenshot from Account > Preferences > Categories.
So the Discussion and Meta categories first posts will show in unread for me. Follow up topics will not…unless I choose to watch a topic.
GG is muted. I need to read the category manually and posts won’t show in Latest or anywhere else.
Developers, How To etc will show in Latest for me, and I can choose to read them or not.
Finally — I have the default set globally for the site so that new users are “watching” the Discussions category — which is essentially most like a single topic mailing list.