The employer's support letter, the original credential the profession requires, proof of Canadian citizenship, and any agency or client agreement that explains where the work will physically happen. A short résumé and the offer letter help the officer connect the pieces.
Bring the record behind every sentence in the letter
Officers read the support letter and then look for the documents that make each statement true. A letter that names the Engineer profession should be accompanied by the licence or degree it relies on, in original form where possible, with a certified translation if the credential is in French. The letter itself should state the job duties, the length of the assignment, the pay and the address where work will be performed.
If the Boischatel applicant will be placed at a client's bridge sites under an inspection contract, carry the contract or a summary letter so the officer can see that the U.S. employer, not the client, directs the work. Proof of citizenship means a passport, not a driver's licence.
Keep everything in one folder, keep copies at home, and do not present drafts that differ from the signed version; a discrepancy between two documents invites questions about which one is true.