USERDEFn.用户定义usern.用户；使用者user-level communications用户级通信user-to-user signalling用户对用户信令user perceived availability用户感知的可用性User Functional Description用户功能描述User current interest views用户最近兴趣视图user-definable vocabulary个人数字助理（PDA）user-selectable baud rate用户可选波特率user' s behavior predictionuser-friendly improvement人性化改进user-based security model基于用户的安全模型