On the lower levels are features, use cases, and supplementary specifications.
较低层次的是特性，用例和补充规约。
It is clear how supplementary specifications affect the use-case flow of events.
清楚补充规格说明如何影响用例事件流的。
Supplementary specifications describe requirements that affect multiple use cases.
补充规格说明描述了影响多个用例的需求。
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