Power of Attorney (POA) is a legal document giving legal authority to another person to operate your account (in this case the Demat account) as per the instructions contained. POA, though not a mandatory document to be signed, is required for smooth operation of your Demat account. Whenever you sell shares from your demataccount or want to pledge them for getting margin for trading, the POA is used to debit the shares from your Demat Account.