SWIFT ISO 15022 - Code SSNX Meaning Social Security Number: Number assigned to an individual by a social security agency. Usages Message type Field number Tag MT574.IRSLST (IRS 1441 NRA-IRS Beneficial Owners' List) 19 95a (Party) MT574.W8BENO (IRS 1441 NRA-Form W8-BEN) 16 95a (Party)