Internal documents that predate the petition carry the most weight: the written protocol itself, its revision history, training material, and records showing how few people are qualified on it.
Contemporaneous records beat descriptions
Hypothetical example: The strongest evidence is material the company created for its own purposes rather than for the petition. A drying-schedule manual with dated revisions, quality records showing the results the protocol achieves, a training matrix listing who is certified on which process, and internal correspondence about developing the method all establish that something proprietary exists and that the employee is connected to it. Confidentiality agreements and any registered intellectual property support the claim that the knowledge is not public. A letter written by a manager for the petition is necessary but should summarize documents that are attached, not stand alone. Where records are thin, it is better to acknowledge that and explain the practice honestly than to overstate what exists.
Gather original internal records before drafting the support letter, including controlled manuals, training lists and dated project materials. Hypothetical example: An indoor-farming equipment maker wants to move the employee responsible for its nutrient-sensor design calibration process. A letter may describe the process, but the contemporaneous calibration notes and restricted training materials show both how she learned it and why it is not ordinary horticultural skill. Redact genuine confidential material carefully rather than replacing all proof with conclusions.
Index the exhibits by the fact each one proves. That prevents confidential technical material from being submitted without a clear connection to the claimed knowledge.