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This is necessary for credit forms, because it is “just” an invoice with a negative amount, usually the inverse of an invoice that has to be rectified. This is a requirement for electronic invoicing too. However, it is not the only use case: sometimes it is necessary to have some lines with a negative quantity, even if the total is positive, to emphasize discounts or to register expenses, like bank commissions in autoinvoices. Part of #61.