sales.xml contains all kinds of total objects such as discount, subtotal, grandtotal, tax that are displayed on the cart page or on the details page of the order in the administrator and in many other places or in others. I must say everything related to the total in the quote or the order defined in This file.
You can add your custom total object also using this file. See the link below for more details
How to add a fee to order totals in Magento 2
Let me know in case you need more explanation about this 🙂