I don't know for sure because I have extra time so am in a smaller room, but my friends in the regular hall have had two types of seating arrangement. In the first set of exams, apparantly they were given a slip of paper with a seat number on it at the door as they walked in and that was their assigned seat. Attendance is taken based on a slip of paper for us that we fill in before the exam starts with our exam number, signature, etc. so it doesn't actually matter where you're sitting. The other times, they've been given a seating plan on the outside of the room that they check before they go in, a bit like in GCSEs and A Levels. This is the same way I get my seat number in the smaller room.