Internal training records, authored documentation, dated project histories, and evidence of how few people in the group hold the same knowledge. Acquisition history is part of the proof.
Show the years, not just the title
Hypothetical example: Because the classification depends on knowledge that is the company's, the file should trace where the knowledge came from. Useful records include internal training curricula and completion dates, technical documents or procedures the beneficiary wrote or maintains, commissioning reports from earlier projects, and any patents or proprietary designs the beneficiary worked on. Equally persuasive is scarcity: an organisational statement showing how many employees can perform the function and why.
A Selwyn firm should also keep the reasoning about local hiring concrete — what a competent local engineer would need to learn, from whom, and over what period — because a general assertion that the person is hard to replace carries little weight. All of this is easier to assemble while the projects are running than to reconstruct in the month before filing.