This tool lists a history of previous functions and allows you to undo a number of functions at once.
Once activated the Undo History form appears; click on the function you wish to restore to.
The drawing will change to show a preview to the restored function. Press Close and the drawing will be restored to this state.
If you have not performed any other operations after the Undo History tool has been used, it is possible to use the Edit>Redo function to repeat any of the operations you have undone.
Some operations cannot be undone - see Edit - Undo for details of the limitations.
Related topics: Edit - Undo, Edit - Redo.