According to the Internal Revenue Service, also known as the IRS, a Taxpayer Identification Number contains nine digits. The number is used by the IRS to enforce tax laws. Taxpayer Identification Numbers begin with the digit "9."
The IRS states that a Taxpayer Identification Number must be present on all tax-related documents, such as tax returns and tax statements. When filing a tax return, a Taxpayer Identification Number is also required for all parties claimed as an exemption. In most cases, a Social Security number acts as a taxpayer identification number, but if a Social Security number is not available, the dependent must have an Individual Taxpayer Identification Number. The number is primarily for non-residents and resident aliens who do not qualify for a Social Security number.
The IRS states that the Social Security Administration issues Social Security numbers on Form SS-5. All other Taxpayer Identification Numbers are issued by the IRS.