as per the U.S. Deparment of State website...
A visa does not guarantee entry into the United States, but allows a foreign citizen coming from abroad, to travel to the United States port-of entry and request permission to enter the U.S. The Department of Homeland Security, U.S. Customs and Border Protection (CBP) officials have authority to permit or deny admission to the United States, and determine how long a traveler may stay. At the port of entry, upon granting entry to the U.S., the Department of Homeland Security, US immigration inspector, provides you a small white card, Form I-94, Arrival-Departure Record in your passport. Visa Waiver Program travelers receive Form 1-94W. On this form, the U.S. immigration inspector records either a date or "D/S" (duration of status). If your I-94 contains a specific date, then that is the date by which you must leave the United States. Your Form I-94, or I-94W is a very important document to keep in your passport, since it shows your permission to be in the U.S. Review information about Admission on the CBP Website. Also, see Duration of Stay.
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