If you want to sponsor someone for a family-based green card, you need to provide proof of a U.S. domicile in your Affidavit of Support (Form I-864). This guide will explain how to prove domicile when filling out Form I-864 ("Affidavit of Support").Consular processing support without the high price tag. Form 6166 is a letter printed on US Department of Treasury stationery certifying that the individuals or entities listed are residents of the United States. This guide will clarify what domicile is, how to establish it, and the documentation you need for proof of domicile. Utility bills with your name and home address. Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States. What does Pennsylvania domicile mean?