It should name him personally as the professional, describe the surveying services and their duration, state that the U.S. firm directs the work, and record that his Canadian corporation invoices on his behalf. It should also confirm he holds no ownership or officer role in the U.S. firm.
Write the letter around control, not around payment
Officers accept contractual arrangements for TN professionals, but they look for the substance of an employer-client relationship with a U.S. entity. The letter should therefore be signed by the engineering firm, describe the projects, the professional responsibilities, the reporting line and the expected period, and explain that fees are paid to the surveyor's Ontario corporation for his personal services. The surveyor should add a page listing the corporation, his shareholding and any other business interests so nothing looks concealed. Keep the corporation's Ontario registration and the licence together in the same folder. What must not appear is any suggestion that he will solicit other U.S. clients, hire U.S. staff or run a survey practice from a U.S. address, because those are the marks of self-employment that TN does not permit.
Hypothetical example: a survey partnership invoices through its Canadian company, while a U.S. rail consultant directs a one-project right-of-way review. The first review should produce a chain of records: the U.S. statement of work, the Canadian company's invoice arrangement, the named individual professional and the client contact who approves results. The letter settles the immigration point when it explains that the individual, not an unidentified company, will perform the licensed work and that the U.S. client controls the project outcome. Put the corporate registration and ownership disclosure behind that letter, not in place of it. A common failure is a contract signed only by the Canadian business that never says who receives the work in the United States. Correct that before travel; an officer cannot infer the client relationship from invoices alone.