This one is not an item. The element descriptor resource is a way to store element descriptors in resource files, either at build time or run time. This way, element descriptors can be loaded simply by specifying a resource id. You don't have to fool with cell libraries and their environment variables, also you don't need to coordinate multiple files on delivery. Since everything is stored in the application, you only need to deliver one file.
You can use element descriptor resources for placing complicated elements. Instead of bothering with code to explicitly build the descriptor you need at runtime, simply build the elements in a new design file. The design file will become part of your "source code". You can then create a "ElementDescriptorRsc" in a .r file, and when your application is built the design file will be read and the elements will be compiled as an element descriptor directly into your application. At runtime you can load it when ever you need it. The resources generated are platform independent.
Perhaps your application places cells. Instead of annoying the user by switching out their cell library, simply store the cells in you application, load them as needed. Keeps you out of the headache of making sure the correct cell library is attached. "But we want to be able to use different libraries for different users" you say. Simply compile them into a separate resource file and you get the same results.
The Element Picture Item can use the element descriptor resource to make creating tutorials easy.