The company's own artefacts: the proprietary code library with its revision history, commissioning procedures, fault-diagnosis documentation, internal training records showing who has been qualified on them, and a duty description tied to those specific items. Years in industrial automation, however many, prove the wrong thing.
Point at something that exists inside the company
The strongest files identify objects. Not the employee's expertise in the abstract, but the named library, the numbered procedure, the revision that he authored, the commissioning checklist that carries his initials. Produce the revision history rather than describing it.
Produce the internal training matrix showing which employees are qualified against which procedures, because a matrix with one name against the relevant procedure is more persuasive than any adjective. Add the customer-facing evidence: contracts describing systems rather than staff, and a description of how commissioning is directed by the petitioner even when the work is performed on a customer's site. Then write the assignment description in the same vocabulary, so the receiving role is expressed as deploying these particular assets rather than as providing controls engineering.
Where the company genuinely cannot point at anything proprietary, that is important information about whether the classification fits at all.