Describe what the knowledge does and how it was acquired, and attach documents that prove existence and authorship — cover pages, version histories, training records — rather than the technical content itself. Confidential material can be redacted with an explanation of what was removed.
Prove authorship and rarity, not the recipe
A reviewer needs to be satisfied that the procedure exists, that it belongs to the company, that the employee's knowledge of it is advanced, and that the U.S. assignment requires it. None of those points requires the technical detail.
Attach the manual's title page and table of contents with the body redacted, the document control log showing the engineer as author of successive versions, the training register naming who was trained by whom, and an internal memo identifying which employees can perform each function. Add the customer's request for implementation support in its own words, which shows the demand for this knowledge originated with the customer. If a non-disclosure agreement with the customer restricts what can be shared, say so and describe the restriction.
Hypothetical example: an East Hants shop submits a fully redacted manual with no explanation; the reviewer sees blank pages. The same manual with a one-paragraph description of each redacted section is evidence.