Country of domicile is the country where you make your permanent home. To file Form I-864, Affidavit of Support, the sponsor must be domiciled in the United States.
For U.S. domicile, generally one must be living in the United States (to include the District of Columbia and U.S. territories) and plan to maintain a principal residence in the U.S. for the foreseeable future. It’s okay to move to another state within the United States; the United States continues to be your domicile.
For sponsors working outside the United States or traveling for extensive periods, it can be more difficult to prove U.S. domicile. For more information on country of domicile, read Proving Country of Domicile on I-864 Affidavit of Support.