NAFTA allows certain professionals who are U.S. or Mexican citizens to obtain a work permit without first having to obtain a Labour Market Opinion (LMO). University professors, scientists and physicians are generally LMO exempt under NAFTA, Appendix 1603.D.1
Permanent Residents (Green Card holders) of the U.S. or persons with Permanent Resident status in Mexico are not covered under the NAFTA provisions.
Documentation required at the port of entry is described under "Crossing the Border".
It should be noted that the NAFTA agreement is meant to serve foreign workers who are entering Canada to fulfill a temporary position. The length of the work permit issued is at the discretion of the border officer. Renewals can be obtained in Canada and must be applied for prior to the expiry of the work permit.