The Undo & Redo commands are only available when the project is locked, and they can be used as follows:
Erases the last change done, reverting the project to a previous state.
Reverses the Undo command.
The extended lock is maintained across multiple browsers and devices. For example, if a project manager locks projects or work packages on a desktop and then wants to continue editing on a tablet, the lock is still in place when he starts the tablet session.