She means the kind of soda you get from the fountain (fountain being that big metal box with six or so soda label thingies that you stick your cup under and push the button or switch to dispense).

Regarding the reason diet soda from fountains taste like crap... if you've worked in a restaurant, a convenience store, or a movie theatre--any place they sell fountain soda--you'll know the product is a mix of syrup and carbonated water. I've got a little theory. The better the quality of your local water, the better the quality of the fountain soda will be. If you live in a small city, or around a place where there isn't much industry, the water will be good. I've lived in fast developing places where the water's full of chlorine, and even the non-diet soda tastes like carbonated cough syrup. But in smaller towns, it's good, and even the diet sodas will have a a bit of added sweetness to it.

Well, anyway. I'm sure you didn't need to know all that, but oh well. ^^