Records that predate the petition are worth more than anything written for it. Authored manuals with revision histories, training curricula and attendance rosters, trade-secret or patent filings naming the technology, and escalation correspondence showing the company routing its hardest problems to him all evidence the claim instead of restating it.
Prefer records the company created for its own reasons
Adjudicators see a great many letters composed specifically to support a filing, and they discount them accordingly. Contemporaneous business records are harder to argue with because nobody made them with an application in mind. Pull the document control system for the anchoring specification and print the revision log.
Pull the training folder and show the curriculum, the dates it was delivered and who sat through it. Pull the engineering change requests and show who approved them. Where the company protects the method as a trade secret, produce the register, the confidentiality schedules and the access list.
Then add the forward-looking material: a description of the American assignment written as tasks, the affiliate's supervision structure, and a statement of what the company would have to do if he declined. Two mistakes recur. Petitions attach organizational charts that have nothing to do with specialized knowledge, and they omit the one thing that would settle it, which is proof that the knowledge is the company's and not the trade's.