Looking at the manuals it sounds like you mix up fresh saltwater in the bin and system pulls it into the reservoir in one gallon increments. First it empties a gallon of water to the waste tank then it brings in a gallon from the fresh tank. Personally, I'd be more interested if the system could empty 10, 20, 30 gallons at a time and then refill the tank but to do that it would need to be able to shut down pumps and heaters to avoid running them dry. I think that I'll stick to my 5 gallon buckets.
Mike