I am looking for a word that describes the following type of rule:
Let's say a customer has received a gift card. This gift card has a rule that prevents the customer for shopping anything in the shop, but only specific categories of merchandise. Let's say only books. So the customer may only purchase books and not the other merchandise like CD's, posters, aso.
The shop would then add a "XXX rule" on all gift cards that should be limited for payment on specific categories of merchandise.
Example usage: Customer: "Why am I not allowed to purchase this CD with this gift card?" Cashier: "I am sorry, but there is a (XXX rule) that specifies only books to be bought with this gift card."
I've thought about restriction, but that is, to my knowledge, generally used to describe what to not allow, and the word I am looking for is more to the essence of describing what to only allow
I am not a native english speaker, so my question may contain strange assumptions due to my limited english grammar knowledge.