A number or alphanumeric character assigned to a mortgage-backed security (MBS) by the issuer as an identifier of that security. Pool numbers are typically six digits in length. Different issuers such as Freddie Mac, Fannie Mae and Ginnie Mae use different alpha characters as the initial digit in their pool numbers to identify the pool as their issue. For example, a Freddie Mac 30-year pool number might be D54321 while a Fannie Mae 30-year pool number might be F54321. 

|||Mortgage-backed securities (MBS) traders frequently use pool numbers (as opposed to Cusip numbers) to identify and trade securities as the pool number is more easily quoted, and the initial digit in the pool number frequently identifies the issuer. Those familiar with pool numbers might also be able to quickly distinguish between older and newer issues by the pool number.