The several widgets are supported via widget class specific files, that are located in the ``additionals'' directory. To add support for a new widget, the user has to write such a file. The name of the file is the class name of the widget class to be supported. The user should take a look at the files that have been defined already (e.g. ``elements/Button'') to get an impression of this kind of support files.
In general, it is up to the user what he does in this file. There have to be some procedures that follow certain naming conventions. Besides, the user can implement the support as he wants to. Of course, the existing support files use many functions provided by XF to make the writing of the widget support files easier.