Good question!!!
"Supplement" these days means that the term "a la Carte" has been eliminated, as a LOT of people do not comprehend that French term, of "Only, or One dish, alone"
Supplement these days means that: (and I'm using this as ONLY an example)
You go to Ruth's Chris' Steakhouse:
A Rib Eye is 39 bucks.............PERIOD. That means you get a steak, and THAT IS IT.....
Supplements mean that you have to order "a la Carte", or separately from the menu........
So: Steak, Prix Fixe (that means a fixed price, but STEAK only)
Supplements: baked potato, salad, etc.........BTW, at baked tater at Ruth's is 7 bucks.........I can buy 20 pounds of taters for 3 bucks, and go to our butcher, Tip Top Market, which has Harris Ranch Organic Beef and get 2 Rib Eyes for 35 bucks.....and fix them myself......to my liking......Enjoy!!
Christopher