TCN NIV applications refer to the process, by a “Third Country national (TCN) “, normally a any non-Canadian or non-Mexican foreign national who wishes to apply for a U.S. nonimmigrant visa (“NIV”) from a U.S. Consulate in a country other than the foreigner’s home country. TCNs can also, in theory, apply for NIV visas at ANY U.S. Consulate anywhere in the world, where the U.S. Consulate will accept to process their visa application.
Foreign nationals in the US applying for a nonimmigrant visa at a U.S. consular post in a third country such as Canada or Mexico, or for that matter, anywhere in the world where a U.S. consular post will agree to entertain their NIV application.
Warning: Individuals who have ever been out of status in the United States because they overstayed their visa or their I-94 (even by one single day!!) are not eligible to apply at a border post or a U.S. consulate overseas.