Adding elements (tags) would typically be used only in an in-house situation where both the writing application and the reading application have prior knowledge of the new tags.
只有在内部环境允许的情况下，比方读写两方都对新的标签能够理解时，才会用到新元素(标签)的添加。
The meeting started in the White House Situation Room, in the basement of the West Wing.
会议在白宫西侧地下室的情况室召开。
The meeting started in the White House Situation Room, in the basement of the West Wing.
会议在白宫西侧地下室的情况室召开。
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