Equipment-specific training records and certifications, a description of the processes involved, the employer's statement of need, and evidence about the availability of such workers.
Specialisation is proved with training records and process detail
Generic praise is the weakest evidence available. What works is specific: certificates for the particular control system or machining platform, records of manufacturer training courses with dates and hours, apprenticeship and journeyperson documentation, and a description of the actual parts, tolerances and processes the employee handles. Alongside that the employer should explain why the role matters to the enterprise's efficient operation, what the consequences of leaving it unfilled would be, and what efforts have been made to find the skills locally.
Where the skills relate to proprietary tooling or a customer's qualification requirements, say so and provide the underlying documents. Because the classification anticipates that essential skills are often needed for a defined period, include a knowledge-transfer plan naming who will be trained and over what timeframe; it shows the request is measured rather than open-ended.