The employer's letter. It has to establish the listed profession, the professional-level duties, the temporary period and the employer identity in one place, because the officer will not assemble those facts from other papers.
Write the employer letter as if it will be read alone
Hypothetical example: a U.S. timber-management company sends a Mission forester a one-paragraph offer that names her as a forestry supervisor. The title is not on the list and the paragraph says nothing about duties.
The fix is not a longer résumé; it is a letter on company letterhead that states the position as Forester, describes harvest planning and silviculture responsibilities, states the requested period and salary, explains why a professional forester is needed, and is signed by someone with authority. Bring the degree or registration evidence, proof of citizenship and, where applicable, evidence of any minority ownership. Foreign-language documents need complete certified English translations.
A letter that matches the credential alternative the applicant relies on removes most of the questions an officer would otherwise have to ask. Hypothetical example: A Canadian occupational therapist is offered a rehabilitation role by a U.S. clinic whose letter gives a start date but no worksite or supervisor.
The first review produces a document list: passport evidence, a detailed letter, credential records and any licence evidence required for the work. The decisive record is usually the employer's signed description of professional duties and the temporary assignment, not a résumé assembled by the applicant. Confirm that the clinic, rather than the applicant, controls the service relationship.
If the licensing process is incomplete, describe its actual status; do not imply authority to practise that has not yet been granted.