A secondary credit number(SCN) is a nine-digit number that has the exact same genetic makeup of a Social Security Number. It is simply an available file number at the credit bureaus that can have financial information and payments reported. You have the right to establish this number only once, so do not abuse it. This explanation is not here to advise anyone to misrepresent your Social Security Number, as you are completely responsible for any debts you incur using your Secondary Credit Number. -Wikipedia
How Is It Legal?
SCN's are used by celebrities, congress members/government workers, and witness protection reasons. It is your legal right to keep your Social Security Number private, and use a separate number for any credit related purposes. You are only required by law to disclose your Social Security Number to the Internal Revenue Service, your employer, when registering a motor vehicle, buying a firearm, or applying and obtaining a federally-insured loan such as FHA, Sallie Mae, etc.