If your're a paying member you can refer to any thread you've posted in. When you're a member, you get two buttons that link to threads you've created, and threads you've chimed in on.
"Tag" is just posting for the sake of that convenience. If there's a hot-button thread you want to keep tabs on, but dont really have anything to say, members post and say "tag" to set up the system and to indicate they are watching, the thread's worth following.