From FlyerGuide Wiki

Emerald Club is the frequent-renter program from National Car Rental. It is one of the more popular car rental clubs for the following reasons:

• All members can choose their own car from the Emerald Aisle on arrival rather than being assigned a car - this speeds renting, allows more flexibility, and caters to personal vehicle tastes
• The program allows airline miles to be earned instead of free car rental points
• Members can quickly attain "Executive" status, allowing a wider selection of luxury cars