Contemporaneous internal records: configuration and revision histories, design documents, authored training material, project files, adaptation records for equipment, and any registered intellectual property.
Prefer what already existed
The persuasive material is almost always the material that was created for operational reasons rather than for a petition. Revision histories that name the employee, commissioning records for a system built in-house, an internal manual they wrote, and purchase files showing standard equipment was modified to a specification all speak without adjectives. Registered intellectual property helps where it exists, and its absence is not fatal.
Ask the technical lead to assemble this before anyone drafts prose, because the prose should describe the records rather than the records being assembled to fit the prose. Where the record set turns out to be thin, that is worth knowing early, and describing the limitation accurately is preferable to overstating it. Hypothetical example: an aquaculture feed-system designer plans to transfer a specialist who adapted its dosing controls for a proprietary production line.
The first review should result in an evidence index: design revisions, access records, commissioning reports, training notes, and a plain-language explanation of what each item proves. The dated technical files settle whether the asserted knowledge existed in normal operations and was connected to the employee. Preserve them before software migrations or staff changes make retrieval difficult.
A failure occurs when declarations tell a sophisticated story but the underlying documents are undated, generic, or unavailable. Start with authentic operational records, then have the technical lead explain their significance to a reader who has never worked in the field.