The only info I can find on the official 2012 website is below:
Tickets in price category A will provide seating that is generally closer to the field of play than tickets in price category D at the same session and venue. Lower price categories will still provide a great Games experience. Your application is for tickets in particular price categories rather than for specific seats. A number of price categories are available for most sessions. The number of price categories depends on the capacity and configuration of the venue.
and
Your application will be for tickets in a particular price category rather than for specific seats - seating plans will not be available. Most sessions have tickets available in more than one price category. Tickets in price category A will provide seating with a better view of the field of play than a ticket in price category E at the same session and venue. All price categories will still provide a great Games experience.