It's possible to add one project to the classpath of another project. With that, the second project can use/references classed that are part of the first project. 

Very often the referencing project also needs to use the libraries that the first project uses. Currently all libraries need to either be added to the second project manually or through a IDE-wide library definition. 

Manually adding all libraries is a really cumbersome and cluttering the IDE with project specific libraries is also not helpful. Especially if the NetBeans project definition is shared e.g. through Subversion or Git. 

When adding a project as a "library" it should be possible to have an option "Inherit references libraries".