Addressing the challenges faced today during the development of multi-functional system families, we suggest a service-oriented
approach to formally specifying the functionality and, in particular, the functional variability already in the requirement
engineering phase. In this paper, we precisely define the underlying concepts, such as the notion of individual services,
the combination of ... [Show full abstract] services, inter-service dependencies, and variability. Thereby, we especially focus on establishing the
consistency of the overall specification. To that end, we formally define conflicts between requirements and describe how they can be
detected and resolved based on the introduced formal concepts.