Group
An empty container, to organize and move sets of objects.
A Group is an object with no geometry and no properties of its own, which exists to hold others. It appears in the hierarchy as a folder that opens and closes.
What is inside follows the parent in position, rotation and scale. That is what makes a group useful: moving the house moves the windows along with it, and rotating the group rotates everything around the same center.
A group can be created in two ways: from the object catalog, empty, or by selecting objects in the hierarchy and using Create Group in the context menu, which already puts them inside.
Since the group has no geometry, the inspector shows only the transform. The appearance section does not show up either: the ones with a surface are the children, and each one can inherit the surface of the parent.