In most cases, foreign nationals who are not permanent residents require a visa to lawfully enter the United States. However, there are a few exceptions, including automatic visa revalidation. Certain foreign nationals availing themselves to this exception may return to the United States after visiting Canada, Mexico, or adjacent islands for less than 30 days with an outdated visa, as long as they can also show a valid passport, an unexpired Form I-94, maintenance of and intention to resume nonimmigrant status, and that no new nonimmigrant visa application was filed while abroad.
Foreign nationals may seek automatic visa revalidation under these circumstances even after the visa has expired in the same or different visa classification (where USCIS has changed your nonimmigrant classification). If the foreign national meets each of these requirements upon entry, U.S. CBP will automatically revalidate the most recent electronic I-94 or generate a new electronic I-94 with the appropriate period of admission subject to passport validity requirements.
Please note that automatic revalidation is limited to certain nonimmigrant categories. Foreign nationals holding F, J, and M visas are subject to other requirements.