When I was serving up hamburgers for Jack in the Box (my high-school job) "plain" meant a hamburger patty sandwiched between a dry hamburger bun, but the term we were trained to use when a customer ordered it that wasy was "AB", meaning absolutely bare. This was standard throughout Foodmaker, and many other burger places. This was in San Diego, BTW.

FWIW:

"Royal" meant "with everything" (sauce, and pickles on either a hamburger or cheeseburger; sauce, pickles, tomatoes, onions, lettuce, on a hamburger or cheeseburger deluxe).

"DP", for "divided portion" if you wanted the sandwich cut in half.

One could always tell when a Jack in the Box employee or former employee was ordering at the drive through because the order would be punctuated with code and special requests. At our store, special requests weren't considered a pain, it was a departure from the routine and those items tended to be fresher because they were usually prepared to order.

The best restaurant food codes and/or slang come from short-order cooks and waiters/waitresses who work in diners/coffee shops.