The coupon availability rule can be used to decide when and how a coupon can be used. For example, you may not allow the coupon to be combined with other coupons or you may want to limit the number of times a coupon can be used by the same user.
The coupon availability rule can also use XSL transform to determine whether this coupon is available for use. For example, you may restrict the coupon to a single use per customer or the coupon should not be allowed to combine with another coupon. You can also restrict the coupon to members only. The expected output should return "true" to indicate this coupon is available for use under the input conditions, otherwise "false" if disallowed. The Storefront comes with several pre-defined rules that you can simply modify the values without needing to know XSL. To learn more about XSL, please see the XSL Transform section.