According to ezmonee's post a long time ago, there are like 3 tiers to the Avis program (maybe a 4th). I don't remember all the details, but here's the gist of it:

Risk: Avis owns the car outright and tries to squeeze every nickel out of it possible.
Tier: Avis has a buyback program with the auto manufacturer for set time/mileage intervals where they can turn it back in for specified dollar amounts.
Flat: Avis owns the car for a pre-determined mileage/time frame. At that time, it's bought back by the auto manufacturer.

If a car is in a tier/flat category and has more than a specified amount of damage, Avis cannot turn the car back in and it either becomes a Risk car or is sold. The same goes to cars that go over their time/mileage allotment. You'll probably notice that vehicles with higher mileage are Risk vehicles. The R/T/F can be found on a round sticker near the barcode on the back driver's side window or on the keychain.

Note: If you search for ezmonee's post, you can get much more info on the categories. I apologize if any of the info isn't 100% accurate as my memory isn't perfect!