The system device is the top of the node and below that is the bus and adapters that have a dependency on the higher-level system device. The bottom of the hierarchy are devices where no other devices are connected. These devices have dependencies on all devices above it in the hierarchy. Parent-child dependencies are used at boot time to configure all devices that make up a node. Device Classes Managing devices requires the operating system to comprehend what device connections are allowed.
Functional classes consist of devices that perform the same function.
Printers, for example, comprise a functional class. Functional classes are grouped into subclasses according to certain device similarities. For example, printers have a serial or parallel interface. Serial printers are one subclass and parallel printers are another. Device types are classified according to their model and manufacturer.