This might seem light a dumb question....but....are the "bottles" on a fuel belt to hold water, electrolyte drink or gu?

I guess I have never been able to figure this out. When the IMSG came through everyone seemed to have one with something in the bottles. So if they are used for drink etc. how often are they used if you have an aid station +/- every mile or so?

Why are the bottles smaller? Do you refill at aid stations?

Thank you for answering my crazy questions. This might solve the other question of How useful are they? Inquiring minds want to know

Red Rock