If set, this marks a point in the task's activity stack that should be cleared when the task is reset.

设置这个标志意味着在activity栈中做一个标记，在Task重置的时候栈就把从标记往上的activity都清除。

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The root activity in the stack is the one that began the task — typically, it's an activity the user selected in the application launcher.

栈中最底部的是任务的起始活动——一般是用户在滑出的程序列表中选择启动的活动。

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When the user presses the BACK key, the current activity is popped from the stack, and the previous one resumes as the running activity.

当用户按下BACK键后，正在运行的活动被弹出栈，旧一个恢复成为正在运行的活动。

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