To browse subjects in an information catalog:
The Subjects window opens, displaying a list of objects in your information
catalog. These objects contain other objects, but are not contained by
any other object. The Subjects window opens in an icon view by default,
but it can also open as a list view.
The tree view shows you the relationship of the objects that belong to a particular grouping. The objects in the tree view have plus signs (+) next to them to show that all objects in this view are grouping objects that contain other objects.