A service is an I/O Kit entity, based on a subclass of IOService, that has been published with the registerService method and provides certain capabilities to other I/O Kit objects. In the I/O Kit’s layered architecture, each layer is a client of the layer below it and a provider of services to the layer above it. A service type is identified by a matching dictionary that describes properties of the service. A nub or driver can provide services to other I/O Kit objects.