The knowledge must be of the petitioning company's own product, process, technique or procedure, or an advanced level of knowledge of its processes, rather than expertise the wider industry shares. Length of service alone does not establish it, and neither does being the only person currently doing the job.
Ask what the company knows that its competitors do not
The test is comparative, and the comparison is with the field rather than with the rest of the payroll. Being indispensable to a small employer is a staffing fact, not a legal one. What counts is knowledge that is genuinely proprietary or advanced within the organization: an internal formulation, a method developed in-house, an unpublished protocol, a system the company built for itself.
Hypothetical example: the substrate work of Thanh, a process technologist at a plant-propagation nursery, qualifies on its face because the formulation is the nursery's and is not published. The petition should say what makes it the nursery's, how long it took him to acquire it and how it is protected internally. Hypothetical example: a frost-protection specialist at a cranberry operation is proposed for transfer because he is the only person who programs the bog's freeze-alarm controllers, but the controllers are a widely sold commercial product and the training is available from the manufacturer.
That is valuable industry skill and it is not specialized knowledge.