First Right of Appropriation gives you the right to tell your bank/building society how you want the money paid into your account to be used.
You may want to do this if money is due to be paid into your account and the account is in overdraft or there are bills or other payments due to be paid from it. You can instruct your bank to not use specific payments made to your account. You can also ask that certain payments made in to your account are used to pay specific bills.
Further information and a sample letter are available from National Debtline.