Requirements engineering plays a critical role in developing software systems. One of the most difficult tasks in this process is identifying functional requirements. A critical problem in many projects is missing requirements until late in the development cycle. In this paper, our core interest is function modeling, which refers to building models of systems based on their functionalities and on the functionalities of their subcomponents. We present a framework as the basis for specifying functional requirements via a modeling language that produces a high-level diagrammatic representation. The aim is to deliver an overall system description, facilitate communication and understanding, construct a holistic view of the system above the domains of different expertise, and lay the foundation for the design phase. We analyze the notion of function and its elementary types and apply examples of natural language description and scenarios. The results reveal a new method that lays a foundation for works on functionality and viable methodology for capturing its requirements.