Discounts
Two ways to represent Zid discounts in Odoo, and when to choose each.
Discount calculation type
Either the discount is applied to the order lines themselves, or it is added as a separate negative line representing the discount.
Discount product
If you chose the separate line, pick the Odoo product that represents it. Create a service product with a clear name such as "Discount" so it is easy to follow in your reports.
Which one to use
A separate line is easier to account for and shows total discounts clearly. Discounting the lines gives a more accurate margin per product. Choose based on what your reporting needs.
Change this before orders pile up
Changing the method does not rewrite old orders, so you end up with two different representations of discount in Odoo. Settle this early.