USERDEFn.用户定义usern.用户；使用者user-to-network interface用户网络接口User identity confidentiality用户身份保密性user friendly database system用户友好数据库系统user interface design pattern用户界面设计模式User Logistic Support Summary用户后勤支援总结user/session identification用户／会话识别User Datagram Protocol UI Unix生存时间用户数据报协议User cancelled installation.用户取消了安装。user-specified image content截图内容User's guide to the dictionary