This button enabled display of a segmented view of the source of a compilation unit.
This button is only shown if you
customize your perspective to show the Editor Presentation actions.
For example, if a method is selected in the Outline view, the Show Source Of Selected Element Only option causes only that method to be displayed in
the editor, as opposed to the entire class.
The entire compilation unit is displayed in the editor, with the selected Java element highlighted in the marker bar with a range indicator.
Only the selected Java element is displayed in the editor, which is linked to the selection in the Outline or Hierarchy view.