Records created before the transfer was contemplated carry the most weight: training logs, validation reports, procedure documents naming the employee, project records and correspondence about the process in question.
Contemporaneous records beat letters written for the file
A letter from a director asserting that the employee is uniquely knowledgeable is expected and discounted accordingly. What corroborates it is the ordinary paper the company already generated: the training matrix showing who was certified on what and when, validation or commissioning reports the employee signed, revision histories on standard operating procedures, and internal messages where the employee was the person consulted. Supplier and customer specifications the employee worked to can be included where confidentiality permits, with sensitive commercial detail handled sensibly. Also gather evidence that both entities are doing business and stand in a qualifying relationship, since that requirement is identical to L-1A. Any French-language document submitted to USCIS needs a complete certified English translation.
Hypothetical example: a specialty tea blender claims knowledge of a confidential batching method, but the file contains only a manager's praise letter. Gather ordinary evidence with dates: restricted training records, procedure revisions, project assignments, access logs, and a concise statement from the person who taught or supervised the method. The first review should pair every asserted skill with a record and identify what may be redacted without obscuring the point. Decide whether the remaining documents show a process tied to this employer rather than a standard trade skill. Confidentiality is not a reason to omit all corroboration; it is a reason to organize it carefully.