Some plugins, such as SideKick and Console, offer services, which can be described in a file called "services.xml". Here is an example of one from SideKick:

<SERVICES>
        <SERVICE CLASS="org.gjt.sp.jedit.buffer.FoldHandler" NAME="sidekick">
                new sidekick.SideKickFoldHandler()
        </SERVICE>
</SERVICES>

For information about services, refer to the ServiceManager class API documentation. There, you can find out what the tags and attributes mean, as well as how to register and use services.