Something that has a security number is a security.
Security means surety, which means "one (as an accommodation party) who promises to answer for the debt or default of another." Security/surely is a debt instrument.
The 'social' security number is for security. What is the security? The person/entity/citizen JOHN JONES. If John of the Jones acts as agent for JOHN JONES then John now promises to answer for the debt or default of another.
Don't be the surety. Don't use the security. Don't offer its number. Get the f away from it. It's their security. Their person. Their franchise. Don't be the master/mister/captain of their U.S. Citizen Ship.
Surety