Knowledge of validation methodology in general is professional skill. Knowledge of the company's own protocols, why they were written the way they were and how to apply them across sites is specialized knowledge. The petition must show the second.
The company's system, not the discipline
Hypothetical example: a Rothesay supplement manufacturer's validation specialist has a science degree and industry certifications that many quality professionals hold. Those establish that he is qualified; they do not establish specialized knowledge. What does is his authorship of the company's cleaning-validation protocols, his knowledge of the assumptions and regulator interactions behind them, his familiarity with the company's equipment and product matrix and his experience training staff at the Canadian plant to follow the system.
L-1B specialized knowledge is knowledge of the petitioning organization's product, service, research, equipment, techniques, management or other interests and its application in international markets, or an advanced level of knowledge of its processes and procedures. The petition should describe the protocols, show they are proprietary, and explain how long a newly hired quality professional would take to acquire equivalent command of them. Comparing him with other staff and explaining why only a few hold this knowledge helps the officer see the distinction.