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. Examples of acceptable residency documents include: Current, unexpired PA driver's license or photo ID card.PA vehicle registration card. Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support. Bring two of the following items as proof of residency in Pennsylvania: Current utility bills (for example, electric, cable, telephone, etc. Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States.