A record is kept of the last 32 changes you have made to the model diagram, including operations like adding new elements, deleting, moving and renaming elements, and entering or changing a value or equation.
The Undo button on the toolbar and the Undo command on the Edit menu reverses the effect of the preceding operations in turn (up to a maximum of 32). You must undo operations in sequence, it is not possible to undo an earlier change without also undoing all subsequent changes.
The Redo button on the toolbar is, in effect, an un-undo button: it reverses the effect of a preceding click on the Undo button. As with the Undo button, you can Redo any number of Undo's (up to the maximum of 32). However, note that the Redo button is only activated immediately after a click on the Undo button. If you click on Undo, then do something else (like add a new compartment), then the Redo button is greyed out and you can no longer use it.