Numerous systems engineering (SE) methods for the model-based and textual specification of systems focus on managing complexity solely by partitioning the system based on physical structures or by defining different views of the system and therefore reach their limits in agile development. The increasing demand for an agile system development requires an agile systems engineering procedure for the model-based and textual top-down specification of systems. Although function-based development, This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.