The States / Parts panel lists all States and Parts that exist for a Component, Module, Device Panel or Device Panel Group. When Parts are added to the Canvas such as images or rectangles etc, they appear in the panel under the current selected State. When in Module mode, Parts can be Components. When in Device Panel mode, Parts can also be Modules and when in Device Panel Group mode, Parts can only be Device Panels.
Components can have many States, where as Modules, Device Panels and Device Panel Groups only have one State and that is called Background.
The Canvas will only show the Parts that have been added to a particular State. When in Component mode, there are two options in the View Menu that allow Background State Parts to appear on all States and another option that allows all Parts to appear in all States. These options are to assist in building Components and do not impact the final look and implementation of the Component.