Accountability is the whole point of the audit trail, so every entry names who did it. The userId links the entry to the User who carried out the action — whether that was creating an artifact, changing a project's status, or reclassifying material under NAGPRA. When you read the audit history for a record, this is the field that lets Relic show you a name rather than an anonymous change. It is captured automatically from the authenticated session at the moment the action happens, so the attribution reflects who was actually signed in.
Technical section
Data model, API, and internals — visible to developers.