Debit Note & Credit Notes are used when there are transaction regards to Debtors & Creditors.
For instance A Supplier has issued an Invoice to his Debtor. Now he realizes that the rate charged in the Invoice shud have been 150 instead of 140. So in this case the supplier has undercharged the customer by Rs.10. So in this case the Supplier will issue a Debit Note to his customer, stating the facts of the case & will debit the customers account with the difference.
In turn the debtor will record the difference in his books & he will send an Credit Note to the creditor stating that his account is being credited by the difference.
In this way there are numerous other transactions for which Debit/Credit notes are being issued by the either party.