This menu item comes available when a project is selected in the tree view. It allows to free up disk space by deleting Document Revisions and labels from the selected project. The Document Revisions are marked for deletion and can be removed permanently by the Purge action (to be executed by an administrator). However, labels are removed permanently immediately by the Clean up project action.
Note
Back up your data before you clean up a project.
More information is available in the Help of the Clean up dialog.