Dated artifacts from the company's own systems: version-control histories, design specifications, patent or invention records, validated procedures, training materials and internal staffing data. Letters from managers should point to those records rather than replace them.
Let the company's own archive make the case
Adjudicators are persuaded by records that were created for business reasons rather than for the petition. Hypothetical example: a Sainte-Brigitte-de-Laval simulation engineer at an industrial-automation firm is proposed for a U.S. plant commissioning.
The strongest exhibits are the version-control log of the firm's proprietary control-logic library showing her commits over four years, the validation reports she authored for two previous plants, the internal training curriculum in which she is listed as the sole instructor for that library, and an organizational chart identifying the three staff who can deploy it. Add the U.S. client's contract showing the plant uses that library, a technical director's memo quantifying training time for a replacement.
Draft the support letter last and make every claim in it traceable to an exhibit; a letter that describes the knowledge as unique without a document behind it is the most common reason for a request for evidence.