A deeper, slower file-health check than Purge - Revit's Audit command scans the entire project database for low-level corruption and inconsistencies that build up over long project lifespans, especially in heavily workshared models, and repairs what it can. It is typically run when a file starts behaving strangely (unexplained crashes, elements that won't delete cleanly) or as periodic preventative maintenance on large, long-running projects.

Audit and Purge solve different problems - Purge removes unused content, Audit repairs unhealthy file structure - and a well-maintained large project usually runs both on a regular schedule rather than waiting for something to visibly break.