Payroll registers, employment contracts and variation letters, human resources leave records, tax slips, benefit statements, and travel records. Where a reorganisation moved the person between entities, corporate records showing the relationship between those entities also matter. Assemble these before writing any narrative summary of the history.
Go to the source systems early
Payroll and human resources systems are often retained for a limited period, and records from a predecessor entity can be harder to retrieve than expected. Request them at the start rather than when a question arrives. Where a record cannot be found, note what was searched and what was available instead, so the gap is documented rather than silently smoothed over in a summary.
Request the records in writing and keep the request, because the response time is itself information the plan needs. Where an archived payroll system is involved, or where a predecessor entity held the records, expect weeks rather than days and expect a retrieval charge. Ask the human resources owner at the outset how far back the current system holds data and what happens beyond that point.
A gap discovered at the start can often be filled from tax slips, benefit statements or the employee's own copies; the same gap discovered during preparation tends to be filled with a narrative instead.