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Any snapshot must conform to the type diagram associated with the use case it illustrates. Snapshots are useful as an aid to clear thinking during analysis and design. They represent instances of use cases. Although simple ones may be included in formal documentation, We usually recommend against this. After drawing them, the type model can usually be updated. Note in particular that neither the action post-condition nor the snapshots say anything about the sequence
of events; they only describe the outcome, regardless of the precise sequence of events leading to it. This is important when we come to the design of reusable components because two components (or two business units) may implement the use case quite differently. The interface to a reusable component should only specify the outcome and the vocabulary, not the sequence of implementation operations that leads to that outcome.