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 Segments, Fillers and groups.
Buttons which allow you to add call lines to your Segment:
Other buttons allow you to remove existing call lines from the Segment or move them up/down in the list.
Sections opens in the right part of the window, depending on the Element you click on in the list: