Debit Note is issued by seller when the buyer owes money to the seller. A seller issues debit note when the value of the invoice increases.
If seller has made mistake while issuing invoice to buyer and value of invoice increases after correction of error, seller issues debit note to buyer.
Eg: Sales Invoice was issued for Rs.1000 (10 units * Rs.100). If revised price is Rs.110, debit note of Rs.100 (10 units * Rs.10) would be issued. Buyer is liable to pay Rs.1100 (10 units * Rs.110) to Seller.
Read more - What is Debit Note under GST?