For I-130s filed by a U.S. citizen (“petitioner”) on behalf of a relative (“beneficiary”) who is not physically present in the U.S.:

• Petitioners “residing” in the U.S. should mail their petition to either the Chicago Lockbox or the Phoenix Lockbox, depending on their state of residence.
• Petitioners residing in a country with a USCIS office may choose to file and have their case adjudicated there, or file with the Chicago Lockbox. China, for example, has two USCIS offices. The USCIS Beijing Field Office accepts I-130s for petitioners residing in Beijing, Shanghai, Tianjin, Hebei, Jilin, and the Northeast (Heilongjiang, Jilin, Liaoning, and Shanxi). The USCIS Guangzhou Field Office has jurisdiction over the remainder of China. For petitioners with this option, our firm usually strategizes with the client to determine where filing the I-130 will be fastest and have the highest odds of approval.
• Petitioners “residing” in countries without USCIS offices should normally also file with the Chicago Lockbox.
• In the past, U.S. Consulates abroad in jurisdictions without USCIS offices routinely accepted and adjudicated I-130s but that service has been discontinued. Now, a Consulate can only adjudicate an I-130 where it “requires immediate processing due to exceptional circumstances.” In that case, the Consulate will contact USCIS to ask whether the Consulate may accept the case. Examples of exceptional circumstances include cases involving U.S. armed service members, medical emergencies, threats to personal safety, cases close to aging out, certain adoption cases, cases where the U.S. citizen petitioner has been given short  notice related to transferring to the U.S. or taking a new job there, etc. The petitioner’s place of residence is no longer relevant.

Note for that this purpose the term “residence” means the place of general abode; the place of general abode of a person means his principal, actual dwelling place in fact, without regard to intent.  The USCIS offices in China don’t tend to philosophize about whether a U.S. citizen “resides” in the country or is just temporarily present as a visitor, student, etc. Instead, these USCIS offices focus just on whether the U.S. citizen has a residence permit, work visa, student visa, or other long-term official authorization permitting residence in China.

Proving the U.S. Citizen Is “Domiciled” in the U.S.
An applicant for an immigrant visa based on an approved I-130 is required to prove that he or she is unlikely to become a “public charge” in the U.S. The term “public charge” means primarily dependent on the U.S. Government for subsistence in the form of public cash assistance for income maintenance or institutionalization for long-term care at U.S. Government expense. This usually requires the U.S. citizen to file a Form I-864, Affidavit of Support (although some exceptions apply). The affidavit is the petitioner’s contract with the U.S. Government to provide support to maintain the sponsored immigrant at an annual income not less than 125 percent of the Federal poverty income line. In the contract, the sponsor also agrees to reimburse any federal or state agency that provides a means-tested benefit to the sponsored immigrant.
A key issue for expats is that to file the Form I-864 the citizen must be “domiciled” in the U.S.  The concept of domicile is related to residence, as defined above. The key difference between residence and a domicile is that only the latter requires an intention to remain in the place for the “foreseeable future.” For example, a student who attends an out-of-state college is resident in the dorm, but she isn’t domiciled there if she intends to return to her home state after graduation.
There are several ways to prove you are domiciled in the U.S.:

Arguing That You Are Temporarily Abroad but Domiciled in the U.S.
Here’s the State Department’s guidance to its officers about determining that an applicant is temporarily abroad but “domiciled” in the U.S.:
Some petitioners have remained abroad for extended periods but still maintain a principal residence in the United States (i.e., students, contract workers, and non-governmental organization (NGO) volunteers). To establish that one is also maintaining a domicile in the United States, the petitioner must satisfy you that he or she:
(1) Departed the United States for a limited, and not indefinite, period of time;
(2) Intended to maintain a U.S. domicile at the time of departure; and,
(3) Can present convincing evidence of continued ties to the United States.
Evidence that a trip abroad is temporary may include, for example, proof of your voting record in the U.S., proof of paying U.S. state or local taxes, proof of having property in the U.S., proof of maintaining bank or investment accounts in the U.S., proof of having a permanent mailing address in the U.S., proof that you are a student studying abroad, or evidence that a foreign government has authorized merely a temporary stay abroad.

There are also two specific situations where persons abroad temporarily count as domiciled in the U.S.:
Domicile for Permanent Resident with Approved Form N-470, Application to Preserve Residence for Naturalization Purposes
A permanent resident living abroad temporarily pursuant to the terms of an approved Form N-470, Application to Preserve Residence for Naturalization Purposes, is considered to be domiciled in the U.S. To file a Form N-470, a person must be physically present in the U.S. for one continuous year after being granted permanent resident status and must contemplate being abroad for more than one year for purposes of employment for certain U.S. employers, public international organizations, or religious organizations.

Domicile for U.S. Citizen Working Abroad for a U.S. Employer
A U.S. citizen living abroad qualifies as “domiciled” in the U.S. if his or her employment abroad meets the following requirements:
1. The citizen is employed by one of the following types of entities:

• the U.S. Government;
• an American institution of research recognized as such by the Attorney General
• “an American firm or corporation engaged in whole or in part in the development of foreign trade and commerce of the United States, or a subsidiary thereof”;
• a public international organization in which the United States participates by treaty or statute; or
• is a minister, priest, or missionary; and

2. The citizen is “regularly stationed abroad,” meaning that he or she “proceeds abroad, for a period of not less than one year, pursuant to an employment contract or orders, and assumes the duties of employment.”
3. The citizen is living abroad “temporarily.” (This is a circular argument, isn’t it?)

Reestablishment of Domicile
If the sponsor is not domiciled in the United States, the sponsor can still sign and submit a Form I-864 so long as the sponsor satisfies the government by a preponderance of the evidence, i.e., providing “concrete evidence,” that the sponsor will establish a domicile in the United States on or before the date of the intending immigrant’s admission or adjustment of status.
USCIS and the State Department list several factors that may show an intention to reestablish domicile in the U.S.:

• Accepting a job or “seeking employment” in the U.S.
• Signing a lease, purchasing a residence, or making arrangements to stay in another’s home. (Just beginning a house search is not mentioned but may be relevant too).
• Registering children in U.S. schools.
• Opening a bank account or transferring funds to the U.S. or investing in the U.S.
• Voting in U.S. elections.
• Getting a U.S. driver’s license is not mentioned but the U.S. Consulate in Guangzhou has found it to be relevant in some cases.
• This list of factors is not meant to be exclusive. A sponsor may have other relevant evidence of plans to reestablish domicile in the U.S.

In addition, evidence that you are severing similar ties abroad may bolster your claim that you are moving back to the U.S.
An immigration officer must later deny the intending immigrant’s application for admission or adjustment of status, if the sponsor has not, in fact, established a domicile in the United States on or before the date of the decision.