In Odoo, the "Open: Order Lines" window is used to view and manage the details of the order lines within a sales order. When you click on the section header of a sales order, this window is opened and displays the details of the order lines within that section, including information such as the product name, quantity, delivery address, and other relevant details.

You can use this window to manage the order lines within a sales order by adding, editing, or deleting them as needed. This can be useful for making changes to a sales order, such as updating the quantity of a product or changing its delivery address.

It is normal for the fields in this window to be empty when you first open it, as they will only display data once you have added order lines to the sales order. Once you have added order lines, you can click on the section header again to open the "Open: Order Lines" window and view the details of the order lines within that section.