Answer
A credit note is a source document reducing the amount a customer owes—issued for returns, overcharges, damaged goods, or allowances. To the issuer (seller) it's a sales return; to the receiver (buyer) it's a purchases return. It's often printed in red and recorded in the relevant returns day book.
Real-world example When goods are returned, the seller issues a credit note that the bookkeeper enters in the sales returns day book.