The undo and redo commands navigate a stack of actions from a limited set of commands (and the corresponding Toolbar icons):
Up to 10 undo/redo actions are remembered. Other actions/commands cannot be undone, including surface, volume, label, and many others. In addition, after a selection change is undone, using select down or the keyboard down arrow to navigate the selection cascade may not work as expected, although undo/redo can still be used to navigate the previous selection states.
The same stack of actions can be navigated by using the Edit menu or the keyboard shortcuts shown in that menu (platform-dependent). The Edit menu also shows the type of action that would be next undone, with entries grayed out when no actions are available to undo or redo.