Multiple views of requirements are needed to sufficiently understand or describe the behavior of a complex system. For example, you might use object-oriented analysis (OOA) to assess the real-world entities relevant to an application. OOA will help identify them and understand their interrelationships and attributes. You might use finite-state machines to describe the desired behavior. You might use decision trees to describe the system's behavior in response to complex input, and so on.
Reference:
Yeh, R., "Software Requirements: New Directions and Perspectives," Handbook of Software Engineering, New York. 1984.
