If you are a U.S. citizen wishing to sponsor your foreign citizen relative for a green card, you may (or may not) need to file, among other things, a Form I-864, Affidavit of Support. This form is intended to create a contract between the U.S. government and you in which you (the sponsor) give a financial guarantee to make sure that your relative (the beneficiary) never becomes dependent on U.S. government assistance for his or her basic needs.
In order to qualify as an I-864 sponsor, you must (among other things) be "domiciled" in the United States. This means that the U.S. must be the country where you currently maintain your "principal residence" (the place where you live most of the time) and where you intend to maintain such residence for the foreseeable future.
If you live abroad but still claim a U.S. domicile, you will need to submit with your Form I-864 a written explanation and documentary evidence in support of your claim. Your claim will be successful only if you can prove either that are employed abroad by certain U.S.-related entities or that you are otherwise living abroad temporarily.
U.S. citizens who live abroad will be considered U.S. domiciled if they are employed by one of the following U.S. related entities:
In some (though not all) of these cases, "employment" could include not only full-time, permanent employment but also work under grants, contracts, or other forms of services, and there is no strong requirement that such employment be the only (or even the main) reason for the employee's presence abroad. The definition will depend on the particulars of your case. (Consult an immigration attorney to find out if it applies in your case.)
Letters from employers (on official letterhead) should be submitted as evidence. (Employees of U.S. companies should also submit evidence of their company's creation and ownership.)
U.S. citizens who live abroad will be considered U.S. domiciled if they left the U.S. only for a limited period of time (however extended such period may be) with the intent (at the time of departure) to maintain their domicile in the U.S., and if they have also kept ties to the country (after their departure).
Immigration officers recognize that many U.S. citizens who are not employed by U.S.-related entities still move abroad with the intent to pursue definite projects and to return to the United States at some later date. These often include students (such as participants in study abroad programs), contract workers, and nongovernmental organization volunteers. In order to prove that they have maintained a domicile in the U.S., such persons should emphasize the term of their project, as evidence of their intent to maintain U.S. domicile at the time of their departure from the United States.
Evidence that ties to the U.S. were maintained could include receipts for storage facilities in the U.S., proof of renewed licenses, subscriptions or contributions to organizations in the U.S., proof of voting in U.S. elections, proof of property in the U.S., proof of visits to family and friends in the U.S., proof of receiving mail in the U.S., proof of having renewed a U.S. driver's license, and statements of bank-, mortgage- or other accounts maintained in the United States.
Other evidence could include evidence of temporary (as opposed to permanent) immigration status in the foreign country, as well as relevant correspondence with institutions, colleagues, family, and friends.
If you are a U.S. citizen domiciled abroad, you may still be allowed to file a Form I-864 if you intend to establish a domicile in the U.S. in the future; either before or at the time of your beneficiary's arrival in the United States. Evidence of such intent would include proof that you have taken concrete steps towards leaving the foreign country to move permanently to the U.S., such as correspondence with U.S. landlords, resignation from your foreign job and application to jobs in the U.S., initiation of a business in the U.S., closing of accounts with foreign institutions and opening of accounts with U.S. institutions, and so on.