By anchoring the claim to the employer's own systems. Knowledge qualifies when it is special knowledge of the company's product, service, equipment or techniques and their application in international markets, or an advanced level of knowledge of its internal processes. Excellence at a widely practised occupation, however rare, is a labour-market fact rather than a qualifying one.
Ask what the organization would lose, not what the market would pay
Hypothetical example: Anouk wrote the calibration logic behind a diagnostics group's own laboratory platform and commissions it wherever a new site adopts it. The most reliable way to test a claim is to imagine the position vacant tomorrow. Suppose the group had to commission a client laboratory without Anouk: could a competent technologist from another of its sites be brought up to speed in the ordinary training period, or would the project stall because the calibration rules and their exceptions live partly in her head?
The second answer describes specialized knowledge, and it can be evidenced rather than asserted. Useful evidence includes the internal documents she authored, the change history of the calibration logic, records of her being escalated to when other sites hit failures, and the training curriculum she built for colleagues. Two cautions.
Long service is not the argument; a technologist with fifteen years of general duties has tenure, not specialized knowledge. And the knowledge must be the petitioner's own, not a third-party instrument vendor's, or the claim belongs to that vendor rather than to her employer.