The statement of work, the governing services agreement, the project plan, the reporting structure on both sides, and any description of deliverables. Add internal material showing what the employer developed and how the individual works with it. Correspondence describing the arrangement in commercial terms should also be reviewed.
Read the commercial papers as an outsider would
Contracts written to satisfy a procurement department often describe people as interchangeable resources at a daily rate. That framing is normal commercially and unhelpful here. Flag the clauses covering supervision, deliverables, and pricing, and let counsel say which matter. Where the wording cannot be changed, it is better for the file to acknowledge it than for it to appear unexplained beside a letter describing something different.
The design and test record with authorship identified, internal procedures and work instructions, training material, any patent or trade-secret registers, and payroll records that establish continuous employment. Discuss training records, permitted project descriptions, internal qualifications and other evidence that accurately explains the knowledge, and ask the company to identify what can be shared and what requires special handling. Build a dated review file around training originals, project versions, supervisor confirmations, entity relationship records, and source files showing personal participation.