Clients can delete objects. This does not mean that they actually get removed at that moment. Instead, they are marked as deleted, and hidden from the normal view. (The menu item View | Show Deleted Items displays them again.) Objects marked for deletetion can be actually removed by choosing from the File menu. Purging means that all removable items are deleted from the database. Purging is irreversible. Once an item has been purged, it cannot be recovered, except perhaps via external means such as a backup of the underlying database. To avoid that clients purge items by mistake, only the administrator is allowed to do this.
Purging will not necessarily remove all objects that are marked for deletion. Items that are somehow marked as 'useful' by the ITP/MDK Repository will not be removed. This is for example the case when a user deletes a source document, but not the model that has been created from it. The source document will not be purged until the model itself gets deleted too. The exact reasons for not purging an item can be rather intricate, so we will not elaborate on them here.
The Administrator, however, has a special sub node 'purging blocked by' that lists the objects that would prevent the revision from being purged. It may either be used by these objects, or be configured in them (in case of folders and projects). The administrator can see this node under revisions and base objects marked for deletion, if he has enabled the option Show Deleted Items in the menu View.
Note
Every revision of a document is a complete version of that document, taking up the same amount of disk space as the original document. Deleting a revision and purging will delete that revision from the database and free up some space.