They are called RPO Codes or RPO Numbers

Actually, they are letter/number combos.

In the old days, a car would come down the line with stickers on the windshield. Each sticker indicated some item that was to be installed on the car.

Do a GOOGLE search for RPO Codes and you will find a lot of info...specific sites for Camaros, Corvettes, etc.

Note that if a RPO code is used, it is across the entire GM line. For instance, if RPO Code "DUH" is a power tissue dispenser on a Corvette, it is also a power tissue dispenser on a Vega.