@Darkrai I'm still new at this, but I think Thread is for posts like Lemmy/Reddit (a top-level post and then threaded comments beneath) and Microblog is for Mastodon/Twitter-style posts (short, singular posts that people can then reply to).
From what I've seen, using 'Add New Link' or 'Add New Article' goes to Threads, whereas 'Add New Post' is Microblog. Haven't tried the others yet!