You can set the following preferences in the Undo category of the Preferences window.
To return to the default factory settings, choose in this window. Note that this resets the preferences in every category in the Preferences window.
Select On if you want to be able to undo operations. This is the default.
Select Infinite to perform an unlimited number of undo operations. This option can use a lot of memory. Select Finite to limit the number of undo operations you can perform (specified in the Queue Size box). The default is Finite.
If Finite is the selected Queue setting, specify here the number of undo operations you can perform. The default is 50.
When this option is active, all consecutive time/playback commands are considered a single undo step. (This is the default).
Disable this option if you want each time change to have a separate undo step.