I am not certain but I think you can only use the amount equal to the number of persons on the reservation. For example, and like I said, I could be wrong, my family is 3 adults. I would only be able to charge for 3 adult meals at each order. However, I do not believe there is a time frame in which you can not use another round of 3 credits.
I do not believe the receipt or the card have a list of how many people are on a reservation but I am pretty sure I read somewhere that this was the case because they do not want you sharing your food plan. On the other hand we have had people offer to "pay" for our meal because they were leaving and were not going to use their counter service meals, and the CM at the register did not have a problem with this.