If you click on those little triangular buttons, more hierarchy_parent interactors are revealed and in some cases they can be opened and closed. This parent is designed to show how easy it would be build a file-browser type application using SubArctic. This demo also uses a "panner" interactor to automatically add scrollbars as necessary.
The heirarchy_parent interactor is quite old at this point and probably needs to just be completely redone. The naming of the "special_child" (the one with the decoration) is highly questionable at this point but it has just stuck.