Service System refers to the group of entities that work together to implement a service. An important challenge for the service designer is to organize her conceptualization of the service in a way that helps her identify the functional components required to engineer the service. At a basic level of conceptualization, the functional relevance of an observed category can be historically inspired, empirically informed, rationally thought, or pragmatically focused. From a service-design perspective, the interest in a given phenomenon is limited to its functional organization, which requires taking an exclusively pragmatic view of the world. In this paper, we propose a role-based approach to modeling categories, which requires service-designers assign functional and non-functional roles to categories by making explicit their interpretation of the conceptual relevance of these categories. Staying aware of the design choices will help the service-designer develop an informed model of observed reality, leading to better alignment between the scope and the purpose of the design activity.