StatusNotifierItem: Each application that wants to utilize status notifiers registers its own StatusNotifierItem to the session bus.

StatusNotifierWatcher: Service used to keep track of every active instance of StatusNotifierItem, used by StatusNotifierHost to get the list of all items. It also notifies it when items are added and removed.

StatusNotifierHost: An application that wants to provide a visualization of the existing StatusNotifierItem instances must register a StatusNotifierHost in the session bus.