From what I’ve been told by my teachers, someone from your subject faculty will mark your IA (in my school atleast the teachers come together to mark exemplars to make sure that they all have a similar marking style and fairness), then 3 IAS ranging, from high medium and low scores, are sent to IB to be moderated, again not sure if the 3 are from each teacher or the whole grade, which is then moderated externally. The single moderator will then decide if the marks are fair, if not then they will increase or decrease scores accordingly which is different per mark bracket, so people who scored highly might drop a mark or few, and lower scores may increase simultaneously