That is the amount consumed in a given time that is listed at the top of the dialogue box. If you dont fill in the initial amount it will hold up to 1000000. If you want a realistic use of a product, you work out how often you have to deliver to keep some in hand. It is handy to have an initial amount that gives you time to make the first delivery. then work out how much you use in the time it takes to make the next delivery( thats where you use the consume amount)