Thnkzdaz.
Posted by: dazarobbo
If you look at the (mostly) older forums, you'll see that you usually have the ability to edit the original title of a thread and "branch off" into a different discussion. halo.bungie.org is a good example of that I think.
So, you could probably infer that each "post" (don't think of it as just the text, but rather an id, the text itself, which thread it belongs to, etc...) also contains its own subject field, it's just that the web team has disallowed it to be modified. IIRC, you can look up someone's post history and it will tell you the subject of the thread to at the time it was posted. The fact that the subject of the thread can change per page based on when the 25th, 50th, 75th, etc... post is made, I think means that when that post is made, it's simply taking the current title of the thread and putting it into the subject field of the new post.