Yes, sophiegirl, you are confused.

The Emerald Aisle and frequent-renter Executive Selection allow one to make a reservation from a limited number of classes, then chose a vehicle when you arrive. If you can live with mid-sized or full-sized class cars and like variety, this is the way to go. Even then, some travelers note that pickings at the wrong place and wrong time of day can be pretty slim.

Emerald Reserve is for vehicle categories like lux and SUV that likely don't get the same turnover. They will assign you a specific vehicle.

If an SUV isn't something you need, you might take your luck in finding one if you qualify for Executive Selection (12 rentals/year). SUVs have often been available when I grab a car. But if you have six people and luggage for a week you don't want to be stuck with a Chevy Malibu in one of five colors. I hope this helps. The old fleet-only Tauruses really are dispiriting after a while.