Answer

Planview Hub 18.3 and later

Clicking Process All Artifacts will make it such that upon the next change detection interval, a full scan will be run. This means that all artifacts in the collection will be scanned for synchronization. Additionally, Hub will mark all artifacts in the collection as 'changed,' which means Hub will skip the step of computing the changed fields and process the event as if all fields on the source artifact has changed, to ensure that any relevant fields are updated. This can have performance impacts and/or place a high load on the external repository.

Planview Hub 17.3 - 18.2

Clicking Process All Artifacts will only process artifacts that have previously been synchronized.

You can learn more about the Process All Artifacts button here.