Volume List
The Volume List shows all currently mounted volumes.  The volumes selected in this list will be examined when the Diagnose, Repair, or Snapshot buttons are clicked.  

Click on a volume in the list to change its selection state.  If the list is active, you can use the Select All menu item from the Edit menu (⌘A) to select all volumes in the list.  Make the list active by clicking in it or by using the tab key.  The list will have a dark border around it when it is active.