If the supplier has undercharged the buyer, a debit note
may be sent for the amount of the undercharge.
Debit note is in the nature of a supplementary invoice.
If the supplier has overcharged the buyer, then a credit
note is issued.
Credit notes are also issued to buyers when they return
either goods (for example, where they are unsuitable) or
packing materials on which there is a rebate. Credit notes
are usually printed in red to distinguish them from
invoices and debit notes