Help > Authoring > Dialog Boxes > Edit Window > Tools Dialog Boxes > Compare Window
  
Compare Window
You can compare the document in the current Edit window with another open document. If one document is free-form XML, the other must be as well. In the Compare Document dialog box, choose the name of the open document you want to compare to the currently active document. After you select the document, the Compare window displays changes marked according to the Compare Preferences settings.
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While the Compare command will detect the addition or deletion of graphics and equations, it will not detect changes within a graphic or an equation.