Uniqueness is not stated as a universal, standalone requirement. The relevant analysis concerns whether the employee has qualifying special or advanced company knowledge and whether the proposed work actually requires it, based on the evidence as a whole.
Support the claim with specifics, not superlatives
Identify what the employee actually knows, how it was acquired, and why the U.S. assignment needs it, using concrete training or project records. An unsupported statement that no one else could do the job adds little; a documented, company-specific example is more useful to a reviewer.
Specialized knowledge is defined by reference to the petitioning organization's product, service, research, equipment, techniques, management or other interests, or an advanced level of knowledge of its processes and procedures. Produce a knowledge map linking protected methods, unusual internal systems, training sequence, access level and anticipated US tasks. A vendor course teaches the standard configuration; the employer's implementation departs from that standard in specific, documentable ways, and those departures are where any specialized knowledge lives. Industry certifications are built to be portable and comparable between employers, which is what makes them valuable to the people who hold them and useless as proof that knowledge is special to one organization.