It's a relational database. Each post you make has a bunch of data that goes into a very large table as a row. That post includes the text and title of the post, but it also includes the forum ID, the thread ID (or message ID , the mid= part) and the post ID itself, which is usually shown as p=1. Going to a forum and pressing "post something" creates a row in the database with that forum= id. Posting in forums that aren't on the currently allowed list is not enabled anymore, but it was at one time, so to reuse an existing ID, we have to either move all the old posts out of it, or pick one that hasn't been used. if you just flat out delete the rows in a relational database, bad, catestrophic things happen to your database usually, so we avoid that if at all possible.