It should name the profession as listed, describe the actual surveying duties, state the dates and pay, and identify the credential relied on. A vague letter that only says 'surveyor position' invites refusal.
Write the letter so an officer can tick each condition
Hypothetical example: the firm's first draft says the applicant will 'lead survey operations in the region'. That wording suggests management and independent practice, neither of which helps. A better letter states that the applicant will perform boundary and construction surveying on a named right-of-way project, under the responsible charge of a state-licensed surveyor, from a start date to an end date, at a stated salary, and that the applicant holds a provincial licence satisfying the Land Surveyor listing. It should be on letterhead, signed by someone with authority to hire, and dated recently. Attach the licence, the good-standing letter and a short résumé. If the firm is a client rather than an employer, the letter must still come from the U.S. company and explain the professional engagement. Keep a copy of every version; a revised letter that contradicts an earlier one causes more trouble than a first draft that was simply incomplete.
Hypothetical example: a mapping contractor sends an unsigned project scope and expects the applicant to explain it orally at inspection. That leaves the central facts unverified. Ask for a final letter from the U.S. engager that identifies the worksite, professional duties, supervisor, compensation, temporary period, and the reason the applicant is needed. Put the licence evidence immediately behind it, then the résumé and any project schedule. An opening file audit should assemble a one-page checklist of missing signatures, dates, and inconsistent job titles. If the contract calls the applicant an independent business owner while the letter calls them an employee, resolve that conflict before presenting the package; a port-of-entry application is not the place to improvise the relationship.