It may be special knowledge of the petitioning organisation's products, services or other interests and their international application, or advanced knowledge of its processes and procedures. The knowledge need not be proprietary or unique, but general professional skill alone is insufficient.
The test is ownership of the knowledge, not its difficulty
Hypothetical example: A controls specialist may be genuinely excellent and still not meet the standard, because difficulty is not the measure. The question is whether the knowledge belongs to the organisation. Expertise in a commercially available platform, however deep, describes a person the U.S.
market can hire. Expertise in a sequence the Selwyn firm designed, refined over particular projects and documented internally, describes something the U.S. entity cannot readily obtain.
The petition should therefore be built around what is proprietary: the named internal system, the specific procedure, the research the beneficiary contributed to. Where a role genuinely mixes both — proprietary integration work on top of standard equipment — the evidence should separate the two rather than blend them, because a description that sounds like general engineering competence will be read as general engineering competence.