TD is available to a spouse and unmarried children under 21. A common-law partner is not a spouse for U.S. immigration purposes, so they would need their own basis for admission and cannot rely on TD.
Marital status changes the household's route, so settle it early
Hypothetical example: the surveyor's partner of nine years plans to travel and look after the house during the assignment. U.S. law recognizes legally married spouses; a common-law relationship recognized in Saskatchewan does not create TD eligibility. The partner could be admitted as a visitor for genuine visits, but repeated long stays that look like residence draw scrutiny, and visitor status never permits work. If the couple marry before the move, the spouse can seek TD, may study, and still may not work. Children of the partner who are not the applicant's legal children are likewise outside TD. None of this is a reason to marry for immigration purposes, which is itself a problem; it is a reason to plan the household around the status that actually exists. Ask about the partner's own possible routes, such as their own TN profession, rather than assuming they can follow.
Hypothetical example: an applicant assigned to a floodplain-mapping project wants an adult sibling to accompany the household for the full assignment. A sibling is not a TD derivative, even if the family shares expenses and travel. The practical decision is whether that person has an independent visitor, student, or work basis; the principal's admission documents will not extend to them. For a spouse and children who do qualify, carry proof of the legal relationship and keep their planned stay tied to the principal's period of admission. The first family review should produce separate status plans for every traveler. Problems usually begin when an unofficial family member expects the same length of stay as a dependant without a status that supplies it, or when the household assumes that TD status itself carries work permission: a TD spouse or child may study in the United States but may not be employed there.