Goals provide a basis for reasoning about alternative options . A system goal can be refined through alternative combinations of sub-goals.A fine-grained goal can be operationalized through alternative combinations of system service. The responsibility for a goal can be assigned to alternative agents. Incidental or malicious threats to a goal can be resolved through alternative countermeasure goals. A conflict among goals can be handled through alternative resolution goals.Those different types of alternative are all capture at goal level,and generally result in fairly different system proposals. Goals thus provide the right RE abstraction for making alternative options explicit,exploring their respective implications and making effective decisions based on contributions to soft goals.For example, Figure 7.7 shows two alternative refinements of the goal Avoid[TrainCollision].The first option consists of allowing one train at most on each block;the second option consists of allowing multiple trains to be on the same block provided that a worst-case stopping distance is maintained between successive trains.(Goal-based identification and assessment of alternatives will be discussed in greater detail throughout Parts II and III of the book.)