The -CE lines tab contains the list of elements which compose the current Segment. If no element has ever been assigned to the Segment, this list is empty. If it is not, you can see an ordered sequence of Data Elements and possibly other lines such as group Data Elements, Segments, fillers and groups.
Buttons allow you to add Elements to your Segment:
Others allow you to remove existing Elements or move them up/down from the list.
Sections opens in the right part of the window, depending on the Element you click on in the list: