A bank typically issues a letter of guarantee to inform someone else that the bank guarantees funds or securities for the client. Banking Law Section 96 sets forth the general powers of New York banks.To request a guarantee, the account holder contacts the bank and fills out an application that identifies the amount of and reasons for the guarantee. Bank guarantees and letters of credit differ slightly, but both serve the same purpose: to give confidence and protection during transactions. A bank typically issues a letter of guarantee on behalf of a client who has entered into a contract to buy goods from a supplier. This information should encompass your bank accounts, investments, and other financial assets. A letter of instruction is a document that lists all relevant financial information that your family members or executor need if something happens to you. Free Format Text Details.