Rarity inside the company supports the claim but does not establish it. The test asks whether the knowledge concerns the petitioner's own product, process or procedure, or is advanced knowledge of how that organisation does things, rather than expertise available generally in the field.
Scarcity is evidence, ownership is the test
Two employees holding a skill is a fact about staffing, and a petition that leans on it alone invites the obvious reply that the company could train a third. What moves the analysis is showing that the knowledge is bound to the employer. Hypothetical example: an installer near Saint-Joachim-de-Shefford builds its own load-management logic on top of purchased hardware, and the sequences that keep a site running through a February cold snap were developed in-house over six winters.
Knowledge of those sequences is knowledge of the company's process, and the fact that few people hold it is corroboration rather than the argument. Contrast the same technician holding a manufacturer's certification on a widely sold controller. That certification is portable, other holders exist across the continent, and its scarcity in one small firm says nothing about the petitioner.
A useful internal check is to ask what a competitor would have to buy in order to obtain the same capability. If the answer is a training course, the knowledge is the trade's. If the answer is the company's design history, it is the employer's.