Undo

• drawing tools
• applied effects
• resizing and changing the color format of the image
• changes to the selected area (including selection and deleting of selection)

• restore deleted images
• restore an object after loading another
• deselect a brush color

Redo

You can redo the operations that were cancelled with the Undo command. Restoring operations is only possible if you don't make any changes to the image after undo.