During synchronization, there may be situations where a project was previously located in the source server but is no longer available. The project may have been deleted on the source server or no longer accessible (by the source server administrator's decision).
The synchronization process does not and cannot automatically delete these unavailable remote projects from the target server, because other local projects may still be using this remote project. The missing remote project is not deleted from the target server but moved into a special category - ATTIC.
The target server administrator should inspect this category from time to time. If there are any projects, the target server administrator should inspect whether they are used in the local projects, perhaps using the Project Usage Map.