The object tree
The large white section shown in the upper left of Figure 2 is often referred to as the object tree.
When you create a world, you begin with a background scene (such as the green grass and the blue sky shown in Figure 2).
Then you add various objects to that scene.
As you add each object to the world, an item appears in the object tree that represents that object.
Later on, you can click on an item in the object tree to select that object for a variety of purposes.
(The object tree doesn't change as you switch between the two views.)