That is the post ID which normally increments by 1 with each post created. However, some posts were test posts, others were hard deleted and the numbers haven't always incremented by 1 because of posts that were merged (some manually, some automatically by doublepost prevention) and also missing posts caused by forum software changes; we changed from phpBB to vBulletin some years ago and then this year from vBulletin to xenForo.Where does the number 3,000,000 come from then in the post link? (I am actually a bit confused now!)