All content is organized in method plug-ins. A method plug-in is a container for method packages which, in turn, contain the method and process content. Method plug-ins and method packages allow you to organize your content at a level of granularity that suits your needs for authoring and reusing the content.
When you create a method plug-in, you will usually want to reuse content in other plug-ins. You may want to modify or extend the content in these other plug-ins or add your own content. When you create a plug-in you can reference any number of other plug-ins that want to use in this way. A method plug-in can also be stand-alone and not reference other plug-ins.