A system can be a device that can perform certain functions autonomously, without other devices, but it can also consist of several identical or different devices that complement each other functionally. An example is a computer and a printer. The mechanical components of a system include housings, plug-in racks, subracks and racks. The electronic units are the backplanes, power supplies, and fans, and the functional modules are single- board computers, plug-in cards, and display devices. To make systems comparable, their quality criteria and the interfaces of the functional modules are specified and the products are standardized. This allows users to combine functional modules from different manufacturers.