A Taxpayer Identification Number (TIN) is an identification number used by the Internal Revenue Service (IRS) in the administration of tax laws. It is issued either by the Social Security Administration (SSA) or by the IRS. A Social Security number (SSN) is issued by the SSA whereas all other TINs are issued by the IRS. If you have a TIN then you would enter it in the SSN field.
For more information, see the following:
https://www.irs.gov/individuals/international-taxpayers/taxpayer-identification-numbers-tin
