FinalConfig is an application that implements the preference dialogue (typically found under Edit->Preferences) as a separate application, that can be used by another application (called the client application or client) through DBus.
The available preferences and how they are to be presented to the user is described in a XML-file (the desc file), which can be stored in a place of the client discretion. The client does not need to bother with the desc file (it just needs to pass along the file location when invoking the FinalConfig service).
The actual preferences are stored in a separate XML-file (the config file), which can be stored in a place of the client discretion too. From the clients perspective this is nothing more than a list of key-value pairs.