The soda card is good at all bars and in the dining room. It is not allowed to be used with room service.
You cannot use your own glass or cup. You will be given a bar glass of soda. By the way - on every Carnival cruise we have been on for the past 5 years (which is 5 Carnival cruises), they have used only canned soda. No bar fountains or soda syrup mix. I guess they found it to be easier to just use cans for everything. But - when using the soda card you will not get a full can. They pour a serving into a glass of ice. And don't bother asking for a can unless you plan to buy a can. Frankly we have found the soda card is not a good deal for adults but is a good thing for kids.