Documentation of the system or procedures themselves, the employee's project and implementation history, internal training records, and any organizational material showing how the knowledge is distributed within the group.
The evidence already exists inside the business
Almost every organization holds more supporting material than it realizes: system configuration documents, internal standard operating procedures, project plans and post-implementation reviews, training decks, and the records showing who attended what and when. Gathering it is mostly a matter of asking the right department and obtaining permission to use commercially sensitive parts, possibly in redacted form. Add the employee's own history with those projects, dated. What does not help is a thick file of general industry certifications, which speak to professional competence rather than to the organization's own knowledge. Decide early who inside the company owns this collection task, because letters get written and material does not get gathered when nobody is responsible for it.
Hypothetical example: Sayed worked on a fleet-safety software rollout and the evidence sits in several departments. An opening assessment assigns one person to collect dated specifications, project plans, training materials, access permissions, and the employee's deliverables. Those records establish what the system is and how Sayed came to know it. Preserve confidential material carefully, using excerpts or redactions where appropriate, but do not replace it entirely with conclusory letters. A common problem is submitting only industry certifications, which show competence but not the employer's distinctive process. Arrange the file in the order the knowledge was acquired so a reviewer can follow the development without guessing.