HES standards address both interoperability requirements, interconnectivity requirements and plug-compatibility requirements. Devices are described as interoperable if the functionality of the higher communication layers is such that commands can be exchanged and result in meaningful action. For interconnectivity it is sufficient to specify mechanical, electrical, and functional (lower layer protocol) characteristics such that devices can be connected on a shared transmission medium. Plug-compatibility includes both interoperability and interconnection. It usually implies that a particular interface is claimed for a particular purpose, where showing a certain interface implies interworking capabilities. This is particularly the case in the consumer environment where the availability of a particular plug on two pieces of equipment is generally understood to mean that the devices can be interconnected and will work together. This part of ISO/IEC 14543 addresses the architectural aspects of interfaces by introducing the concepts of Functional Groupings (FG) and Reference Points (RP). These concepts provide a means to describe modularity of a device, and hence serve as a basis for device interface standards