Stage "Select Content"
This stage determines the rectangular region with "useful" or useable content (shaded in color). Why do we need to define this area? Firstly in order to determine the page size to the output. The content will be added to the total margin area, and the outer limit of these margins affects the size of the output file. Secondly so that the final images don't show the line of fold or other debris from the edges. Strictly speaking whether the debris falls in the margin in the output stage depends on the mode. In most modes the margin is filled in white.
If areas are identified incorrectly, you can tweak individual pages manually by setting the mouse pointer over the edge, clicking & dragging as needed. Occasionally Scan Tailor may find non-existent content or conversely not select content where it should. In this case, you can manually create / delete a region by right-clicking on the image, and select the appropriate menu item.