In our understanding, the two ovals represent the possible phenomena involving the environment (to the left) and a programmable plat- form connected to the environment, called the machine (to the right). The intersection represents phenomena at the interface between the machine and the environment. The letters mark specific subsets of relevant phenomena: those which are required (R), those which are not required but nevertheless play a relevant causal effect in the envi- ronment (W), those which concern the desired behavior of the machine at its interface with the environment (S), those generated by the program inside the computer that drives the machine (P), and those occurring internally to the machine (M). This view, considered nowadays as a fundamental model for requirements engineering, emphasiz- es the role of the specification of machine’s behavior at its interface with the environment