Display objects and many of its subclasses are in reality Abstract classes. These classes serve as parent classes for other display objects, combining common functionality into a single class. Hence instances of these classes cannot be created.
Some of the common subclasses of Display Object that are also abstract are
AVMMovie1 – used to represent loaded SWF files that are authored in AS1.0 and 2.0
DisplayObjectContainer – has many useful subclasses such as Loader, Sprite and MovieClip
InteractiveObject – it is the base class for all objects used to interact with the mouse and keyboard.
MorphShape – These are created when you create a shape tween using Flash Authoring. They cannot be instantiated using ActionScript.
Stage - It is created as a singleton instance for every application and is included at the top of the display list hierarchy.