It has to be knowledge of the petitioning company's own product, process or procedures, or an advanced level of knowledge of its processes, rather than expertise the wider trade shares.
Company-specific, not occupation-specific
The most common reason a technically strong case fails is that the evidence describes an excellent tradesperson instead of describing company-specific knowledge. Being the only person locally who can commission a particular class of equipment is a hiring difficulty. Having designed the internal control logic a particular organization runs, and being the person who knows why it behaves the way it does under specific conditions, is different in kind.
The practical test is to ask what the employee knows that a competent outside hire with the same training would not know on their first day, and then to point at the document, system or artefact that difference lives in. If nothing can be pointed at, the case needs rethinking rather than rewording.