Evidence that the knowledge belongs to the employer rather than the trade: internal documentation of the proprietary system, records of the training or development work through which the knowledge was acquired, and a description of how few people in the organization hold it.
Show the thing itself, not a description of expertise
The strongest material is usually already inside the company and nobody thinks to look for it. Internal design standards, the sequence library and its revision history, commissioning checklists that carry the employer's own methodology, training material written for new technicians, and project records showing which installations the specialist personally configured all describe knowledge that is specific to the organization. Add the employment history showing how long the knowledge took to build. A letter that simply asserts many years of experience and a high level of skill describes a good employee, which is not the test. Where a proprietary system is genuinely proprietary, the documentation proving it exists somewhere; the preparation work is finding it and getting permission to use it in a filing.
Hypothetical example: Omar, a dairy-culture formulation technician, has training records but none connect his knowledge to the proposed U.S. work. The file should pair each restricted formulation, internal quality step or unusual troubleshooting method with the U.S. duty that requires it. Versioned manuals, access records and project assignments can show that the knowledge belongs to the company rather than to the industry generally. Corporate records and foreign payroll must separately establish the qualifying relationship and one continuous year abroad. If any work occurs at a customer site, the contract and supervision plan should show petitioner control and that the placement is not labour-for-hire.