Extension of Stay in the U.S.
Many individuals have difficulty understanding the difference between the validity of a visa, which is a stamp placed in a national passport or in a laissez-passer, and permission to remain in the United States.
A visa is granted by the U.S. Department of State, either by the Visa Office in Washington, D.C., or at one of our Embassies and Consulates outside the U.S. A visa is used solely to make application to the U.S. Department of Justice, Immigration and Naturalization Service (INS), to re-enter the U.S. after being outside of this country. That is, a visa is used only to cross borders and has nothing to do with the length of your stay in the U.S. Permission to enter and/or remain in the U.S. and extensions of stay in this country are granted by the INS.
Upon entering the U.S., the INS office at the port of entry places a small white card, Form I-94, in your passport or laissez-passer. On this card, the INS inspector writes in either a date or "D/S" (duration of status). The date on the Form I-94 is controlling. That is, if you have "duration of status" you may remain in the U.S. as long as you are in the same job or same category of job. If you have a date on your Form I-94, you should apply for your extension of stay with INS prior to its expiry.