This shows the objects on a diagram in list form, where you can select/deselect, change tagnames, delete, undelete, and also change the front-back order of the symbols on the current diagram. You can select and deselect symbols so when you go back to the diagram they are selected.
Note - even if an object has been deleted and there were subsequent actions so that Undo would not undelete the object, the object can be recovered until the diagram is closed.
The zOrder is the front to back order. Highest numbers at the front.
See also View Text Only