The Medallion signature guarantee means that Bank of America participates in the Medallion signature guarantee program; this process of verification helps deter fraud on the transfer or sale of securities. Signature guarantees are used for securities exchanges to limit liability for transfer agents in case documents are forged.
The three different Medallion signature guarantee programs include: Securities Transfer Agents Medallion Program, or STAMP, Stock Exchanges Medallion Program, or SEMP, and, New York Stock Exchange Medallion Signature Program, or MSP. If someone isn't a customer, it is within Bank of America's right to decline to guarantee a signature. Conversely, transfer agents can refuse the signature guarantee if they do not recognize the guarantor.