Jeez, you don't "create" bills (since when are WE creating bills that should be created by our supplier?). You get a bill from a supplier and then book it in your system. And when you book a bill, you also have to register the supplier invoice number, so when you pay the bill, you can tell the supplier for which invoice(s) this payment is for. This explanation is so confusing. If you wish to help people understand Odoo, you will have to come up with some real life examples. Not like "oh, we received 5 units and now we create (????) a bill". We do NOT "create" anything which is supposed to be created by our supplier! We are switching to Odoo right now, and I have to admit that the vocabulary used by Odoo developers is extremely confusing, and the logic in the workflows is really strange. It's like clicking on start to turn off your computer. You have to learn to think wrong, to be able to work with Odoo! Once your brain is turned upside down, you'll be able to use Odoo ;)