It should name the listed profession, describe the professional duties in recognisable detail, state the salary, the location and the period of employment, and identify the supervisor. It should also be consistent with everything else in the file.
Draft it with the requirements open in front of you
Hypothetical example: the employer sends a warm two-paragraph letter about how pleased they are and the veterinarian presents it at the border. It answers none of the questions being asked. A useful letter is structured around them: this is the profession, these are the duties that make it that profession, this is the salary, this is where the work happens, this is how long it is expected to last, and this person supervises it.
Ask for a draft and read it against the appendix entry, sentence by sentence. Where a duty sits outside veterinary practice, describe it honestly rather than removing it, since the file has to match reality. The applicant must also be a Canadian or Mexican citizen, meet the Veterinarian entry through D.V.M., D.M.V.
or Doctor en Veterinaria, or a state or provincial licence, avoid U.S. self-employment, and disclose any ownership interest.