This paper presents a unifying reference framework for classifying user interfaces that support multiple targets or contexts of use in context-aware computing. The framework decomposes a context of use into three facets: users, the computing platform, and the physical environment. A context-sensitive user interface is one that can adapt to changes in these contexts. The framework structures the development life cycle into four levels of abstraction: task and concepts, abstract user interface, concrete user interface, and final user interface. These levels are connected through relationships of reification and abstraction, allowing for clearer understanding and comparison of methods and tools. The framework also introduces the concept of multi-target user interfaces and plastic user interfaces, which adapt to context changes while preserving usability. The paper discusses the design and run-time processes of the framework, including the use of ontological models, archetypal and observed models, and the transformation of initial models into transient models. The framework supports both forward and reverse engineering, enabling the development of context-sensitive user interfaces that can adapt to different contexts. The paper also presents examples of the framework's application, including the PetShop tool and the SEESCOA system, demonstrating how the framework can be used to design and evaluate context-aware user interfaces. The framework provides a common basis for understanding and comparing models, methods, and processes in multi-target user interface development.This paper presents a unifying reference framework for classifying user interfaces that support multiple targets or contexts of use in context-aware computing. The framework decomposes a context of use into three facets: users, the computing platform, and the physical environment. A context-sensitive user interface is one that can adapt to changes in these contexts. The framework structures the development life cycle into four levels of abstraction: task and concepts, abstract user interface, concrete user interface, and final user interface. These levels are connected through relationships of reification and abstraction, allowing for clearer understanding and comparison of methods and tools. The framework also introduces the concept of multi-target user interfaces and plastic user interfaces, which adapt to context changes while preserving usability. The paper discusses the design and run-time processes of the framework, including the use of ontological models, archetypal and observed models, and the transformation of initial models into transient models. The framework supports both forward and reverse engineering, enabling the development of context-sensitive user interfaces that can adapt to different contexts. The paper also presents examples of the framework's application, including the PetShop tool and the SEESCOA system, demonstrating how the framework can be used to design and evaluate context-aware user interfaces. The framework provides a common basis for understanding and comparing models, methods, and processes in multi-target user interface development.