A shipping type promotion allows you to offer a discount on shipping during checkout (e.g. free shipping on all products, or free shipping if a coupon is presented). Customers will see the discount applied to the shipping fee during checkout.
The promotion rule can also use XSL transform for complex promotion. The expected output should return the discount amount (a negative value) to apply, otherwise zero if no discount is to be given. 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.