Windows NT has a number of subsystems to control various classes of devices. Classes identify logical device types such as display, keyboard, and network. Because each subsystem has different user options and different compatibility constraints, it is impossible to have a single utility support installation of all possible devices. Instead, each class is responsible for providing its own installer.
A single installer can support installation of both the class and individual devices. However, this depends on the class and in some cases separate device installers are used. Typically, the installer is combined with the Control Panel applet for the given class, although this is not a requirement.
The class and device installers are responsible for displaying the appropriate dialog boxes or carrying out appropriate installation tasks. Although a class installer gives the device installer full control of the installation process, in most cases, the device installer simply directs the class installer to complete the task for it. The device installer can insert additional configuration dialogs, hardware detection, and other features into the process as is appropriate for the device.