Hello all! I'm trying to automatically add a line on customer invoices, with some info of the sale order, but I don't find the way to do it. What I want to do in invoice lines, it's to add a line before the first line of all orders. This line will have only some info in the description field, so the result must be something like this:

description for first sale order (it will be composed with the order number, and a field I have added to sale orders)

lines of first sale order

description for second sale order

lines of second sale order

I need to know which object and wich definition are responsible for adding lines in the customers invoices. Thanks in advice.