如果相同的对象用于多个操作，其可用于追踪哪个操作已完成。
It can be used to track which operation finished if the same completion-handler object is used for multiple operations.
发送操作已完成，但在把消息复制到“已发送消息”文件夹时失败。
Your Send operation was successful, but copying the message to your Sent folder failed.
一旦处理完成，一个响应就会被发送到请求客户机，该响应包含业务操作结果或表明操作已完成的通知。
Upon completion of the processing, a response containing either the results of the business operation or an acknowledgment of the completion of the operation is sent to the requesting client.
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