It must come from the United States employer, describe duties that require the profession, and state the title, salary, temporary period and the applicant's citizenship. Anything that reads as generic project support weakens it.
The letter carries the case, not the resume
Hypothetical example: The letter is the document that connects the person to the profession and to the employment. It should name the Appendix 2 profession exactly, describe responsibilities in terms a practitioner of that profession would recognize, and state the annual salary or wage, the intended period of employment up to three years, and that the position is temporary. It should identify who supervises the work and where it is performed. If a third party is involved, the letter should explain that relationship instead of leaving it to be inferred. A resume supports the letter but cannot replace it, and a job posting written for recruitment purposes is usually the wrong document to submit because it describes the market rather than the agreed position.
Request a signed offer, concise duty schedule, the proposed period and evidence of who will supervise and pay the employee. Hypothetical example: A Canadian actuary is offered an insurance-modeling assignment through a staffing vendor. The first review must determine whether the insurer or vendor is the actual employer and make the letter match that answer. An undated generic letter often fails because it does not connect the professional work, company and temporary assignment in one record.
Keep the signed letter intact.