The employer's support letter, because it is the only document that connects the person, the listed profession and the duties. Everything else supports it.
Build the file around the letter, then check each claim in it
Hypothetical example: A Portugal Cove-St. Philip's applicant should treat the employer letter as a set of claims that other documents must prove. The letter states the profession: the Appendix 2 page proves that profession is listed and states its credential rule. The letter states the applicant meets the credential rule: the baccalaureate degree, or the post-secondary diploma or certificate plus evidence of three years' experience, proves it. The letter states the position is temporary and lasts a defined period: the offer letter or contract shows the term. The letter states the applicant is a Canadian citizen: the passport proves it. Where a diploma was earned outside Canada, the United States or Mexico, add a credential evaluation. Where the letter mentions a start date that depends on a state licence, add the state board's confirmation or the pending-application receipt. An unsupported claim in the letter can lead to refusal at the port of entry.
Hypothetical example: a forester is hired to prepare management plans for a U.S. timber owner. The first review should create a document map: passport for citizenship, Appendix 2 for the listed occupation, degree and transcript for the qualification, and a letter identifying the specific forest-inventory, silviculture, and planning duties. If a transcript uses unfamiliar course names, attach a short institutional explanation rather than expect the reviewer to infer their content. That sequence exposes a missing item while there is still time to obtain it, instead of discovering it during travel.