In a service-oriented architecture, all business services are defined in the business reference model (BRM). The BRM is part of the Enterprise Repository. One of the key principles behind SOA is to break down business services into reusable components that can be combined and shared across the enterprise. These shared components are called web services and they are defined in the service component reference model (SRM) which is also located in the Enterprise Repository. Both the BRM and SRM are hierarchical. The exact structure of the model will be determined at design time.