DDPI stands for Demat Debit and Pledge Instruction. This is a document that allows a stockbroker, like millions, to debit the securities from your Demat account when you sell them and deliver them to the exchange. By submitting the DDPI, you no longer have to enter the CDSL T-PIN and OTP for every sale transaction. However, using the DDPI is voluntary, and you can still choose to sell shares by entering the CDSL T-PIN for each transaction if you prefer.