A resource usage represents any usage of resources in its broadest meaning, for example a usage of a product realized by a resource. <br/>A service usage represents any usage of service in its broadest meaning, for example or a usage of a service that realizes a product. <br/>A Product is realized as one or more Service(s) and/or Resource(s). A Product is created by instantiating a ProductOffering and adding it to a specific customer account. In order to charge a service (or a resource) usage, the service (or resource) has to be associated to a product. <br/>The product usage describes the usage of an instantiated Product.<br/>Theoretically there are many different ways to model resource, service and product usage. For example:<br/>        • As classes that inherited from Usage class.<br/>        • As different states of the same Usage class.<br/>The following Figure describes the first approach where ResourceUsage, ServiceUsage and ProductUsage and its relationships with the specification.<br/><i>Refer to Common Domain guide book section Usage ABE for more details</i>.<br/>