A dated letter naming the listed profession, the professional duties, the period of engagement, the arrangement between the parties and the qualification relied on. Ambiguity in any of those is where questions start.
Ask for specifics that can be checked
A useful support letter states who the employer is, what professional work is required, over what period, and on what terms the applicant is engaged. It should identify the profession as it appears in the listing rather than using an internal job grade. It should name the credential the applicant holds and how that credential satisfies the listing. Where the applicant will spend time at client sites, the letter should say who directs the work. Keep the credential itself, any provincial licence with its issuing body and, where the regulation requires it, a foreign-credential evaluation. Documents in French submitted to USCIS need a complete English translation carrying the translator’s certification of accuracy, completeness and competence, and that translation should render the original rather than improve it.
Hypothetical example: an environmental mapping analyst has a strong degree but the employer's draft letter gives only a payroll title. Ask for a final letter, an identified signatory, credential copies and a short duty-to-deliverable table before travel. The first review produces a document list that lets a reader match each assertion to a record: degree, licence, evaluation where applicable, and evidence of the employer's engagement. If a client site is involved, establish which entity directs the professional work. A contradiction between the letter and a resume is more damaging than a short, candid explanation of a limited duty.