The Employer Identification Number (EIN), also known as the Federal Employer Identification Number (FEIN) or the Federal Tax Identification Number, is a unique nine-digit number assigned by the Internal Revenue Service (IRS) to business entities operating in the United States for the purposes of identification. When the number is used for identification rather than employment tax reporting, it is usually referred to as a Taxpayer Identification Number (TIN). When used for the purposes of reporting employment taxes, it is usually referred to as an EIN. These numbers are used for tax administration and must not be used for any other purpose. For example, an EIN should not be used in tax lien auction or sales, lotteries, or for any other purposes not related to tax administration.