In the document pane, you can see the document which serves as the basis of the questionnaire you are making. This view can be used for example to select the clause you want to apply changes to in the changes pane or locate where a datafield is used that you want to ask a question about (by clicking the button next to the datafield selector in the question options in the card pane).
The button gives you the possibility to take the document back to the Assemble Document mode to edit it.