In the Preferences window, the Undo preference settings let you specify the number of “undos” used by MotionBuilder.
Note: 
You can view a list of all the undos/redos in the undo stack in the Undo window (Window > Undo).
You can view a list of all the undos/redos in the undo stack in the Undo window (Window > Undo).
To access the Undo preferences:
- From the menu bar, select Settings > Preferences.
- In the Preferences window, click the Undo preference option.
The Undo preference settings display.
The Undo stack lets you select an Infinite or Finite set of undos. If you select a Finite queue, you may want to reduce the Undo Level to save memory and speed up MotionBuilder. The default Finite undo value is 10.