This ReferenceTypecan be used to expose the relation between a hardware component and its subcomponent. It indicates that the subcomponent is inside the component. To gain access to the subcomponent typically removing a part of the component (e.g. a hatch) is necessary. For example, it can be used to relate a PC to a PCI board plugged inside the PC. The PCI board is accessible after removing the PC housing. It can also be used to relate a PLC to its CPU, which is hard wired to the PLC, or a network module to its Ethernet PHYs.