DBC识别三种基本的断言类型。
对于同一断言类型，在更具体的目标中设置的策略可以覆盖在高级目标中定义的策略。
Policies defined at high order targets can be overridden by policies set at a more specific target for the same assertion type.
SAML定义了三种断言类型，都是关于用户(人或计算机)的一个或多个事实的声明。
SAML defines three kinds of assertions, which are declarations of one or more facts about a user (human or computer).
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