Documents the company created before the petition existed: training curricula and completion records, authored procedures with version histories, project reports naming the employee, and the record of how long anyone else took to reach the same competence.
Contemporaneous beats persuasive
Hypothetical example: Malachy leads calibration and test for an East Ferris instrumentation maker and has been asked to write a memo explaining why his knowledge is specialized. That memo will be the weakest document in the eventual file. What carries weight is the material already sitting in the company's systems.
The internal course he built, and the attendance records showing who took it. The controlled procedure whose document history lists him as author and approver. Non-conformance investigations closed under his signature.
Customer acceptance certificates naming him as the responsible engineer. The onboarding record of the last person hired into a similar role, showing the months of internal training required before that person worked unsupervised. Where confidentiality prevents disclosing a technique, describe its function and effect and file the surrounding evidence rather than dropping the point.
Collect all of it first; the narrative should be written to fit the documents, never the reverse.