StatusNotifierHost corresponds an object in an application that does the actual graphical representation of StatusNotifierItem instances.
Instances of this service are registered on the Dbus session bus, under a name on the form org.freedesktop.StatusNotifierHost-id where id is an unique identifier, that keeps the names unique on the bus, such as the process-id of the application or another type of identifier if more that one StatusNotifierHost is registered by the same process.
StatusNotifierHost instances don't need any particular property, method or signal registered to the bus. What is important is the presence of the service on the bus, to inform StatusNotifierWatcher that a StatusNotifierItem representation is currently running on the system.