Determines the origin of the invoice/credit memo/pro-forma invoice.
The origin is comprised of a code of origin and the number of the original document.
For a pro-forma invoice, it can be a quote, an order or a delivery.
For an invoice, it can be a quote, an order or a service contract.
For a credit memo, it can be an invoice or a return.
The number of the original document is only entered if the original document is unique.