Functional modeling is in use for the interpretation of the results of model based simulation of engineered systems for design analysis, enabling the automatic generation of a textual design analysis report that expresses the results of the simulation in terms of the system’s purpose. We present a novel functional description language that increases the expressiveness of this approach, allowing a system function to be decomposed in terms of subsidiary functions as well as required effects, increasing the range both of systems and design analysis tasks for which the approach can be used.