When you purchase your tickets OOP you have a choice of Tier 1,2 or 3 with a different price for each. Tier 1 is the most expensive. As far as I understand, seating within the tier is based on when you make your reservation.
For DDP you can only book 2 or 3 and you let then know which you would like. Sometimes tier 2 is booked for the show you request so you can either take tier 3 or select a different show.