This concept can be understood best in the context of an NII model which has received widespread recognition. Each of the three layers sustains a diverse base of technologies, supports a broad base of suppliers, and can continually increase in capability over time.
Services provide data that can be accessed and shared by more than one application.
NII services provide capabilities for electronic creation and diffusion of information needed to satisfy a diverse set of applications. A service will often originate as part of an application then become generic. For example, electronic mail was initially an application. Gradually, the directory, addressing and routing functions separated from the interface and became part of the network services while the interface became the next generation application. Applications will often be built on top of other applications, which in turn, may be composed of still other applications.