The NetBeans APIs are the public interfaces and classes which are available to module writers. They are divided into specific APIs for dealing with different types of functionality. The contents and behavior of the Java source packages and its subpackages, as specified in the NetBeans API List, are the NetBeans APIs.
After you register the NetBeans API sources and Javadoc, you can refer to them in the Source Editor, while developing NetBeans modules.
For the NetBeans API List, see: