Also, you may be referring to the different levels of the Emerald Club. With Emerald Aisle, you pay for an intermediate car and go to an aisle and take any car that is there but you are guaranteed only an intermediate car and thus if that is all that is there, that is what you get.
Another level is Emerald Choice. It works the same as Emerald Aisle as you go to a special aisle and skip the counter. However, with this level you order the particular car you want, like a minivan, and pay the price for that kind of car. You then go to the Emerald Choice aisle and pick up your car and you get the kind of car you did order.