By Jerry Pacheco / For the Journal   When the North American Free Trade Agreement was implemented on Jan. 1, 1994, a new visa also was introduced. The TN visa, often referred to as the "NAFTA visa," was specifically created to allow Mexican and Canadian citizens to work in the U.S. Unlike many other visa categories, the TN visa is a non-immigrant document granting temporary stay in the U.S. for work purposes. In other words, the time spent working in the U.S. for people holding this visa does not count toward residency or citizenship. The visa can be issued for...