这个元素用来通过添加随机选择器，为一个测试运行序列提供随机选择，而不是按照测试顺序地运行。

This element is to provide randomization for a test run sequence, rather than running the test sequentially, by adding random selector.

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优先级列表(实际上是一个指示优先级与选择器的数字)建立了加载处理每个文件数据块的顺序。

A list of priorities, basically a number indicating the priority and a selector, establish the order in which each file chunk is going to be processed by the load.

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