Common components provide shared functionality used among different factory and utility classes.
All the factory classes derive from and depend on the common components so these are the essential part of the entire program. For example, the admin page factory class extends the factory base class. The factory base class is one of the common components. Also the form component which consists of classes to build forms and their data is used by all the factory classes including the factory classes of the admin page, widget, and meta box etc.