X-Setup plug-ins are small files that provide the functionality for X-Setup. Plug-ins have no interface, instead they use the interface of X-Setup to communicate with you. Because of this, they are very small (usually 2 to 4 Kb) and fast. X-Setup itself can't change system system settings; all changes are carried out using plug-ins.
Also, because they are so small, it's easy to update an existing one or to add new functionality by adding new plug-ins.
For example, if you know how to activate a hidden feature inside the program "Super Dub", you can simply create a new plug-in by using the free X-Setup SDK, copy it to your plug-in directory and that's it!
Xteq X-Setup is nothing without plug-ins but simply the best "hacker" tweaking/system administration tool ever with them.
To create your own plug-ins, you will need the X-Setup SDK which can found at the Xteq Systems homepage.