The same documents apply as for any TN case: the factual position description, pay, and intended period from the employer, and qualification and identity records from the applicant. Resolve any inconsistency between the offer and the support letter before it is finalized.
Confirm facts with the party that actually knows them
The Canadian employer should explain the proposed services for the identified U.S. recipient, including the duties, pay and temporary period. Reconcile any client instructions with that description. Keep the domestic job and the U.S. assignment distinguishable so a letter about an ordinary Ontario commute is not presented as the professional entry arrangement.
Where the qualifying path relies on experience rather than a degree alone, that experience needs evidence more durable than a résumé line: dated employment letters describing the actual work performed, or project records naming the person and the period. If the degree was earned at an institution outside the three USMCA countries, obtain a credential evaluation from a reliable service before applying, since the regulation requires it and a port-of-entry officer cannot supply it.