You must first enable the Checkout feature under Configuration > General to access this functionality. The checkout availability rule determines if checkout is permitted based on conditions such as region, amount, quantity, etc.
The checkout availability rule can also use XSL transform to determine whether the checkout should be allowed for complex scenarios. The expected output should return "true" to indicate the checkout is allowed to proceed 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.