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Undo button allows you to reverse most actions. You can undo a series of actions to get back to any previous point in your work by using undo repeatedly. If you go too far you can restore what you've undone with the Redo button or gesture.
The most common use for undo is to erase your most recent ink strokes. You can also use it to reverse an editing operation or recognition that didn't work properly so you can try again.
As an alternative to the Undo button, you can use the Undo gesture:
The Undo gesture is a stroke straight up and then straight down, written away from any current ink on the display.
The undo command reverses actions in the order you performed them. If you did something on tab 1, switched to tab 2, and did something else, then tapping the Undo button repeatedly would undo the tab 2 changes, switch back to tab 1, and undo the tab 1 changes.
Undo can reverse most things you do with the recognizer, such as writing, recognition, and edits, but it cannot undo configuration or preference changes.