The company's own validated process files, revision histories, validation reports and deviation investigations naming the specialist, alongside training materials the specialist produced.
Documents created for business reasons carry more weight than a letter
A support letter is advocacy; contemporaneous records are evidence. The validated process files themselves, with the specialist named as author or approver, are the core. Add revision histories showing decisions the specialist made and why, validation and requalification reports, deviation or non-conformance investigations the specialist resolved, and internal training material the specialist wrote for operators or auditors.
Customer qualification packages and audit responses prepared by the specialist show the knowledge being applied at the boundary of the business. Redact commercially or contractually sensitive material rather than omitting the document. Separately from all of this, payroll records must establish one continuous year of qualifying employment abroad within the three years before the petition, and that requirement is not satisfied by the strength of the technical showing.