An Arrival-Departure Record (I-94 Number) is the admission record that CBP (U.S. Customs and Border Protection) issues to nonimmigrants when they enter the United States at a port of entry. CBP automatically gathers the nonimmigrant visitor's arrival/departure information from their electronic travel records for air and sea travelers. For visitors entering the U.S. through land border ports of entry, their I-94 will still be on a piece of paper.
A CBP officer will stamp the travel documents of the traveler when they arrive with an admission date, admission class, and the date the traveler is admitted until. If a traveler needs a paper I-94, they can request this during the inspection process. Upon exiting the United States, if you have previously been issued a paper form I-94, you should surrender it to the airline carrier or to CBP upon departure. Otherwise, it will be recorded electronically from the flight manifest.