A device component might depend on one or more other devices. A device component depends on another device if the component can be powered off only when all the components of all the devices it depends on are also powered off. For example, the component of the frame buffer device that represents the monitor depends on the mouse and keyboard devices. The frame buffer monitor component can thus only be powered off when both the mouse and keyboard devices are powered off.
The power.conf(4) file specifies the dependencies among devices.