National Insurance number
The National Insurance number is used as a reference number in the Pay As You Earn system, and also by the self-employed. It is also used in applications for Individual Savings Accounts (ISAs), to check that an individual has opened only one ISA in a tax year.
Social Security number
In the United States, a Social Security number (SSN) is a nine-digit number issued to U.S. citizens, permanent residents, and temporary (working) residents under section 205(c)(2) of the Social Security Act, codified as . The number is issued to an individual by the Social Security Administration, an independent agency of the United States government. Although its primary purpose is to track individuals for Social Security purposes, the Social Security number has become a "de facto" national identification number for taxation and other purposes.