A group is a special case of a GO containing a list of GOs and a list of selected GOs – the selection of GOs is local in groups.
Every graphic object is always linked inside a group, if not in the universe then in a special group somewhere else. A group is a graphic object, too; but there are also some special groups called top-level groups which are not linked in any group. They are contained in pages instead. These groups also store the list of a page's mouse-clicks (see Mouse clicks).