This option lets you specify the maximum number of edits that can be undone or redone. For unlimited undo, check Unlimited; otherwise, uncheck it, and enter the desired number of levels in the edit box. To disable undo, set the number of levels to zero. By default, undo is unlimited. Each open document has its own undo history. Undo states may be stored in memory or on disk, depending on the disk threshold. Note that lowering the number of undo levels while a document is open may truncate its undo history.