每个链都可以有一个策略，它定义“缺省目标”，也就是要执行的缺省操作，当信息包与链中的任何规则都不匹配时，执行此操作。
Each chain can have a policy that defines "a default target", i.e. a default action to perform, if a packet doesn't match any rule in that chain.
与规则组一样，选择器拥有日期范围条目、选择条件和缺省目标。
Like a rule group, a selector has date range entries, selection criteria, and a default destination.
XM 使用与这相同的规则， 应用它自己的缺省样式表，而不考虑备用样式表。
XM uses the same rules and applies its own default style sheet instead of considering alternate style sheets.
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