For what there is the sales.xml file? In the documentation, I can not find anything about this file.
sales.xml contains all kind of total objects like discount, subtotal, grandtotal, tax which display on cart page or order details page in admin and many more places. I can say everthing that is related to total in quote or order defines in this file.
You can add your custom total object as well using this file. Check below link for more details
Let me know in case you need more explanation on this :)