Editing : Editing Basics : Multiple Undo : Levels of Undo and Memory

Levels of Undo and Memory
Because Pro Tools needs to keep track of the playlists for all tracks that are edited, the use of multiple levels of Undo can be memory intensive. You can lower the Levels of Undo setting (Editing Preferences) in Pro Tools to reduce the amount of system memory (RAM) used by the Undo queue. If you have plenty of memory available for Pro Tools, you can use a higher Levels Of Undo setting.
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Use the Undo History window to view a queue of undo operations and return to a previous state.
To set the Levels of Undo in Pro Tools:
1 Choose Setup > Preferences and click the Editing tab.
2 Click in the Levels of Undo field and type a value between 1–32.
 
3 Click OK.