Historically both "Advance compaction" and "Client Max Offline Time" were created for both Device Sync configurations, both PBS and QBS. However, after enabling this for PBS, we found that it introduced a lot of bugs, and so we removed it but kept the idea of adding it in the future, it was later dropped, but the docs still reflect it as being for both.

This creates confusion for clients as well as within the team. New hires don't know that this is really only intended for QBS and the fact that from the UI you can enable Client Max Offline time for PBS doesn't help. In this case, enabling this for PBS has no impact.

Docs: https://www.mongodb.com/docs/atlas/app-services/sync/go-to-production/optimize-sync-atlas-usage/#advanced-backend-compaction