Incorporation does not by itself disqualify anyone. TN requires Canadian or Mexican citizenship, a qualifying listed profession with its credential requirements, prearranged professional work, and temporary entry. The disqualifying arrangement is supplying services to a US entity the professional solely or controllingly owns.
Test the arrangement, not the label
A consultant working through their own Canadian company for an unrelated US client is a different situation from one invoicing a US company they control. Independent supervision, a service agreement, or an outside manager does not cure controlling ownership of the US recipient. Confirm the profession and credentials first, then examine the ownership and control facts with counsel rather than assuming either outcome.
The deciding document is whichever one satisfies the qualification rule written into your specific Appendix 2 entry, because those requirements differ from occupation to occupation. Biologist appears in Appendix 2 of the USMCA professionals chapter with a baccalaureate or licenciatura degree as the qualifying credential. Do not invent a universal seniority requirement or assume the title establishes the listed professional activity without supporting duties. A school-district or private secondary-school position falls outside the profession list even when the teacher's qualifications are excellent.
A full membership record from a provincial engineering regulator therefore satisfies the credential element on its own.