Internal documentation of the proprietary system, evidence of Fabien's role in creating it, payroll records for the qualifying year, corporate documents for both entities, and a description of the U.S. assignment including any knowledge transfer.
Contemporaneous internal records beat a well-written letter
The strongest file is made of documents that already existed. Design notes, internal procedure manuals, change logs, training material Fabien wrote, emails approving a pricing model, and screenshots of configuration that carry his name all show knowledge rather than assert it. Add an employer letter that explains why the U.S.
operation needs a person who holds that knowledge and cannot simply hire locally, and how long the assignment will take. Include payroll evidence for the qualifying year, corporate documents establishing the relationship between the entities, and trading evidence for both. Where the placement is at a third party's site, include the contract terms showing that the petitioner supervises and directs his work and remains responsible for the outcome.
Non-English documents need certified English translations, and it is worth checking early which internal material the company is willing to disclose.