Secrecy is not the test. The knowledge must meet the applicable special or advanced knowledge standard, and the other classification requirements must also be met.
Explain the substance
Describe what makes the knowledge relevant to the company and proposed work. Do not rely only on a confidentiality label or a claim that nobody else could perform the task. A description that works usually answers three things in order.
What does the company do differently from others in the same line of work; what did the employee have to learn in order to work within that difference; and what would go wrong if the assignment were given to someone who had not learned it. Answer with facts rather than adjectives. Hypothetical example: a publisher runs a terminology database whose entries carry approval states that control which files may be released, and an engineer unfamiliar with those states will produce output that looks correct and cannot be shipped.
That one sentence does more work than a paragraph asserting deep expertise. Avoid describing the person as irreplaceable, since the standard does not ask for that and a claim of uniqueness invites a comparison the company may not want to make. Keep the whole account within what somebody could support if asked to explain it in a short interview.