The undo and redo commands (and their Edit menu equivalents) navigate a stack of actions from a limited set of commands and manipulations:
Actions performed with graphical interfaces (menus, icons, and tools) will also be undoable if they use the commands listed above. Examples include Build Structure bond rotations and most of the Molecule Display icon actions.
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.
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.