Documents the company itself generated, dated and attributable to the beneficiary: internal procedures he authored, training records, development notes, revision histories and project reports. A résumé and a letter of praise are the weakest possible support, because they assert the conclusion the petition is supposed to prove.
Start from the filing cabinet, not the letter
Work through the company’s own records first and let the support letter describe what they show. Hypothetical example: a CNC programming specialist at a Saint-Cyrille-de-Wendover millwork shop is proposed for the group’s North Carolina affiliate on the strength of a two-page résumé and a manager’s statement that he is the only person who can do the work; the file becomes far stronger once it includes the post-processor he wrote for the shop’s five-axis machines, the internal nesting standards bearing his initials, six years of dated revisions, and the training log showing he taught the method to two apprentices. Keep proprietary material safe by describing its nature and purpose rather than reproducing it.
Add anything that quantifies the training investment — hours, duration, cost — because a factual account of how the knowledge was built is more persuasive than an adjective. Finally, make sure the corporate documents establishing the qualifying relationship and the year abroad are assembled at the same time, since a strong knowledge case cannot rescue a weak relationship.