An I-94 is a form denoting the Arrival-Departure Record of particular foreigners used by the U.S. Citizenship and Immigration Services. Form I-94 must be filled out at the time of entry to the United States by the foreign citizens that are being admitted to the U.S. in some non-immigrant visa status. Citizens of the countries on the U.S. Visa Waiver Program list, who are entering the United States for a short stay and who are not U.S. citizens or Permanent U.S. Residents, must fill out form I-94W instead.
A completed form I-94 (or I-94W) is presented by the foreign citizen at the port of entry to the United Stated to a U.S. immigration officer, who stamps the form and gives the final approval for the admission of the foreign citizen to the United States in a specific non-immigrant status for a specific authorized period. Contrary to a popular misconception, it is not the U.S. visa stamp, but rather the stamped form I-94, that authorizes the alien's stay in the United States in a particular non-immigrant status and for a particular period of time.
As a general rule, aliens departing the United States are required to surrender (typically to the airline or ship representative) their I-94 (or I-94W) forms when leaving the United States, so that their departure can be properly recorded by the immigration authorities.
If I-94 form is lost or stolen while the alien is in the U.S., a replacement form may be requested from USCIS.
While in the U.S., a nonimmigrant alien can apply for a change of nonimmigrant visa status from one category to another or for an extension of the previous nonimmigrant status by filing form form I-539 with USCIS. Upon approving such application, USCIS issues an approval notice, the bottom portion of which is a new I-94 form for the alien, reflecting that alien's new or extended nonimmigrant visa status and also showing a new authorization period for that status.
Travellers who have entered utilizing the visa waiver program may not however change status or extend time of stay in the US. They must depart within the 90-day period allowed visa waiver visitors, and may not simply travel to a contiguous country in order to exit and reenter for a "new" 90-day period.