Audit Trail is recording the information(e.g.login,logout,(insert,delete,update w.r.t. change in system.)) only in the event of changes made w.r.t data.
Also refer to the below answered FAQ
What does an audit trail require to track?
A: The text of the regulation states the audit trail must record "operator entries and actions that create, modify, or delete electronic records." This means that all user data entry, edits, or deletions that modify program data should be tracked in the audit trail
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