The buyer normally issues debit note to a supplier to request for credit note. The buyer may not debit the account of the supplier until his request is approved by him evidenced by the issue of credit note to the buyer.
A debit note is also prepared whenever it becomes necessary for one reason or the other to increase the amount due from debtor. For example, when the seller has under charged a customer on the invoice.
Generally, any expense that should have been charged to the customer but were erroneously omitted when the invoice was made out would be charged subsequently by means of a debit note prepared by the supplier.