Use a comparison that explains relevant capabilities, how they were gained and how they are used. An unsupported ranking such as “top expert” is less useful than a concrete explanation of the work the employee can perform.
Compare like responsibilities
For an industry comparison, describe the ordinary skill baseline and the additional company-related knowledge. For an internal comparison, show levels of process understanding and representative assignments. A knowledgeable manager can explain the basis without disclosing unnecessary personal details about colleagues.
Pair the comparison with selected training and work records, and avoid pretending that all colleagues know less. Organize the evidence around the comparison you have chosen, since the same document can be persuasive in one framing and irrelevant in the other. For a special-knowledge argument, material describing what is distinctive about the organization's approach carries the weight: internal methodologies, proprietary or customized systems, and the ways they differ from what is generally available.
For an advanced-knowledge argument, internal comparison is what matters: how many people hold the equivalent expertise, what the progression to that level involves, and where this employee sits within it. Label each exhibit with the proposition it supports. Where a document cuts both ways — a widely used industry tool that the company has heavily modified, for instance — explain the distinction explicitly rather than leaving a reviewer to resolve the ambiguity unaided.