Most immigrants applying for a family-based U.S. green card (lawful permanent residence) must, as part of the application process, have their petitioner (the U.S. citizen or permanent resident who filed their I-130 visa petition) fill out and submit an Affidavit of Support on Form I-864. (This form is prepared by U.S. Citizenship and Immigration Services (USCIS), and available on the I-864 page of its website.)
The purpose of the Affidavit of Support is to assure the U.S. government that the immigrant is not inadmissible as someone likely to become a public charge -- that is, receive need-based government assistance (often called "welfare"). The Affidavit represents the petitioner/sponsor's promise to support the immigrant financially for a period of years, so that he or she will not need such assistance -- or at least to pay back (reimburse) any government agencies from which the immigrant does claim such assistance.
However, a few lucky applicants are exempt from the Affidavit of Support requirement, and do not need to submit a Form I-864 at all. Immigrants who are exempt include those who have either:
• worked for 40 Social Security quarters in the U.S. (approximately ten years)
• been married while the U.S. spouse worked for 40 Social Security quarters, or
• a combination of the above.
The concept is that a financial sponsor’s responsibility lasts until the immigrant has (among other possibilities) earned 40 work quarters credited toward Social Security. (A work quarter is approximately three months, but it depends partly on how high your earnings are.) So if you have already reached the 40 quarters on your own, through lawful employment -- perhaps while in the U.S. as a student or H-1B worker -- there is no need for the sponsor to fill out an Affidavit of Support for you. And, in an interesting twist, the immigrant can be credited for work done by the U.S. spouse during their marriage.
You cannot, however, just leave Form I-864 out of your application packet and expect the immigration authorities to figure out the reason. You will instead need to include a form called an I-864W, explaining the grounds for the exemption. And, you will have to prove how many quarters of work your spouse or you has done. Contact Social Security about getting a certified statement with this information.
Form I-864W is fairly easy to fill out. In Part 2, you would check the first box.