Tax Residency Certificate (TRC) is the certificate mandatory to claim benefit under tax treaty. Many U.S. treaty partners require the IRS to certify that the person claiming treaty benefits is a resident of the United States for federal tax purposes. The IRS provides this residency certification on Form 6166, a letter of U.S. residency certification.
Form 6166 is a computer-generated letter printed on stationary bearing the U.S. Department of Treasury letterhead certifying that the individuals or entities listed are residents of the United States for purposes of the income tax laws of the United States. In order to obtain this certificate, you must fill up Form 8802, Application for United States Residency Certificate.