The number on the card is also used for tax record keeping. A formula devised by the Internal Revenue Service to determine whether a nonimmigrant is a resident or nonresident for tax purposes. An agreement between the United States and another country to determine how the country's residents will be taxed when temporarily in the United States. A treaty can give certain tax benefits. A note of caution, however.
Tax treaties are very specific.
Just because someone is a resident of a tax treaty country does not mean that the person will automatically qualify for tax benefits. An individual may not be eligible for a Social Security number, which is routinely issued only to U.S. citizens, U.S. permanent residents and nonimmigrants with legal work permission. Anyone without a Social Security number who is required to complete annual income tax returns must obtain an ITIN from the Internal Revenue Service. In that case, the IRS assigns a unique nine digit code for tax-related purposes. The last day to file an annual tax return with the U.S. government if you are a resident or a nonresident with U.S. source income.