It has 2 types mapping: Internal Preference Mapping and External Preference Mapping.
它分为两种：内部偏好图与外部偏好图。
By now this is so well known that internal airlines never even ask what meal preference I have and no stranger sitting next to me orders meat.
目前为止，国内的航空公司从不询问我的食物偏好，我的身边总是坐着食肉的陌生人。
You can also extend the Preferences class to form a MessagingPreferences class, which will include messaging preference for internal users.
也可以扩展preferences类，形成messagingpreferences类，用它包含针对内部用户的消息传递选项。
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