这样做的原因是因为WS - Security规范允许为身份验证附加多个令牌，这样就需要更多的元数据来识别哪一个是主安全令牌。

You need to do this because the WS-Security specification allows attaching multiple tokens for authentication, thus additional metadata is required to identify which is the primary security token.

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示例包括WS - Trust中定义的安全令牌(rst)消息，或者WS - Reliable Messaging元数据中定义的序列消息的创建。

Examples would include security token (RST) messages that are defined in WS-Trust, or the creation of sequence messages that are defined in WS-Reliable Messaging metadata.

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