There is a "Products References" button, what is that ?

You are in the editing view of an invoice or quote. You've chosen the client and the vendor and now you have to fill the list of products. You can add one in two ways, manually by filling each field of each row and add another line, or if you often use a product you can use a reference product previously created by clicking on the "References" tab and save it. Now you can find it by clicking on the "Add reference product" tab. If you want to know more about the reference product you can check the following link : Create reference product