This temporary, non-immigrant visa status is only for Canadian and Mexican nationals working in certain professions. The initial period of stay is 3 years with extensions possible. Although the purpose of stay must continue to be temporary, there is no limit to the total period of stay.
Among the types of professionals who are eligible to seek admission as TN nonimmigrants are accountants, engineers, lawyers, pharmacists, scientists, and teachers.
If the Canadian or Mexican employee is outside of the United States prior to requesting their TN status, no petition to USCIS is required. See the TN Eligibility page for additional details on applying for initial TN status. If the TN employee is already in the U.S. and requires an extension of their TN status, the TN employee may travel home and return to the U.S. to request this extension of stay, or UAMS may submit an I-129 petition to USCIS to request this extension. See the TN Immigration Process page for details on submitting the TN extension request within the U.S.