It must be knowledge of the employer's own product, process or procedure, or an advanced level of knowledge of how that organisation does things. Deep general expertise in drying hardwood, however rare, is skill. The proprietary control logic and the schedule library Tobias built inside this company are what the category is aimed at.
The possessive is doing the work in the definition
Read the requirement slowly and the distinction becomes practical. Special knowledge concerns the petitioning organisation's product, service, research, equipment, techniques, management or other interests and their application in international markets. Advanced knowledge concerns that organisation's processes and procedures.
Both are anchored to the company. So the useful question is not how good Tobias is, but what he would have to teach a competent replacement before that person could do the job, and where that material came from. If the answer is a textbook and two seasons of practice, the case is weak.
If the answer is a decade of in-house development, undocumented reasoning behind particular schedules, and procedures written for equipment this group designed, the case is real. Write the petition around the second answer and resist the temptation to add the first as reinforcement, because it dilutes rather than strengthens.