The corrected support letter, the signed offer or contract, and the credential evidence the profession's listing requires. Keep the written exchange requesting the correction as well, since it shows the discrepancy was identified and resolved rather than overlooked.
Keep the correction trail
If an earlier version of the letter has already circulated internally or to a third party, note where it went and make sure the corrected version replaces it. A file containing two letters with different numbers is worse than one with a single accurate letter. Ask the employer to reissue on a current date rather than backdate the replacement.
The letter should identify the US or foreign employer, professional category, actual duties, worksite, compensation, intended period, and why the applicant meets that category’s precise education or licensing condition. The employer's letter carries the case: it should identify the employer, state that the position is an Engineer position, describe the engineering duties in concrete terms, give the salary, state the intended period of up to three years and confirm the stay is temporary. A degree from outside Canada is acceptable, but it usually needs a credential evaluation showing United States equivalence, and every document not in English requires a complete certified translation.