A complete audit trail captures more than outcomes. For each candidate it records when they applied, which criteria and questions were in use at the time, what they answered, how each answer was scored, any later changes to scores or stages, and who took each decision. Entries are time-stamped and should not be editable after the fact, so the history can be relied on.
When a candidate asks why they were not progressed, or an auditor or regulator reviews a hiring process, the audit trail is the evidence. It also lets teams check their own work, for example by comparing outcomes across groups. Retention needs balancing against data protection rules: records should be kept only as long as there is a lawful reason, with access limited to people who need it.