A credit note is a document issued by a seller to a buyer to indicate a reduction in the amount due for goods or services previously purchased. The main purpose of issuing a credit note is to correct errors of overcharging. This can happen if the buyer is charged more than the agreed price or if there is an error in the calculation of the total amount due. By issuing a credit note, the seller can rectify the error and reduce the amount owed by the buyer. It is a way of keeping accurate records and maintaining good customer relations.