No universal secrecy or proprietary-knowledge requirement applies.
Support the precise knowledge claim
Relevant experience and examples may matter without being secret. Do not equate a confidentiality clause or course price with proof of specialized knowledge.
Procedure qualification records, revision histories naming the author, internal training material, job files and equipment adaptation records, exported before any system migration or retirement. Use training records, permitted project summaries and internal role documentation that explain the knowledge accurately, and have the company identify what needs careful handling before anything is submitted. Name the proprietary protocol, produce its revision history showing the beneficiary as author, include validation reports and internal training records, and show that nobody else in the group has been qualified to run it. Client-confidential material, personal data and commercially sensitive design documents all have to be reviewed before they leave the company, and that review is billed against a volume somebody chose carelessly. What identifies a person in an engineering or service record is narrower: training completions, signed work orders, escalation tickets closed under a login, validation reports countersigned by a supervisor.