Gather the share register or shareholder agreement for every company you hold an interest in, the proposed engagement or offer letter, and any document describing how the two companies relate to each other. Keep the originals rather than relying on a summary written from memory.
Document the ownership before anyone characterizes it
Ask the corporate records holder — usually the firm's accountant or corporate counsel — for the current share register showing the exact percentage held and the date it was acquired. If there is a shareholders' agreement, the clauses about profit distribution and voting matter more than the headline percentage, because they describe what the stake actually gives you. Separately, ask whether the American company and the Canadian firm share any owner, director or contractual arrangement, and get that answer in writing rather than as a verbal reassurance from a manager who may not know. If a numbered company is being used to bill, obtain its incorporation documents too. The point is not to build a large file but to let an adviser see the real structure before the structure is described to anyone else.
Hypothetical example: Elias, a corrosion engineer, finds that a 3 percent holding also carries a voting agreement he had never read. His document index should include the executed voting agreement, current capitalization table, board rights, employment contract and employer support letter, with discrepancies resolved before travel. The percentage alone does not show whether he controls the employer or will work for his own account. Citizenship evidence, the exact USMCA profession and that profession’s required credential remain separate parts of the file. A Canadian may request TN classification at a port of entry or through an employer’s I-129 petition, but neither channel cures an undisclosed control right or permits U.S. self-employment.