With Emerald Aisle, you reserve a midsize. When you get there, you go to the Emerald Aisle row. You can take anything parked there. There's no guarantee there will be an SUV on the Emerald Aisle. All that is guaranteed is a midsize. There are typically full sizes available (like Chevy Impala) but larger are more rare. If you want to guarantee an SUV, reserve an SUV and when you get there, you go to the SUV row.
you don't make an Emerald Aisle reservation for those, unless you like to gamble. You make an Emerald Reserve reservation for that class of vehicle.
It is explained on the Emerald Club page. https://www.nationalcar.com/?action=emcIndex Click on Emerald Reserve.