One of the strengths of the NetBeans platform is its modularity. If you've ever used the NetBeans IDE itself (before, say, reading this book), you've seen this modularity in action when working with plugins: every NetBeans plugin is made up of one or more modules. In fact, NetBeans itself is composed of numerous modules. That's how RCP applications are designed to work. It promotes decoupling, and makes extending and upgrading the application much simpler.
The generally accepted pattern is to, say, put the API classes in one module and the implementations in another. This makes the API classes reusable by other implementers, can help enforce low coupling by hiding private classes, and so on. To keep things simple as we learn the platform, though, we are going to create just one module that will provide all of the core functionality. To do that, we right-click on the Modules node under the parent project, and select Create New Module...: as shown in the following screenshot...