re

剑桥词典

formal
(especially in business letters ) about; on the subject of
（尤用于商务信函中）关于，兹就，事涉
Re your communication of 15 February ... 兹就你2月15日来信…

used in the subject line of an email when it is a reply to another email , followed by the subject of the original message
（用于回复电子邮件时的标题行，在被回复的邮件的标题之前）
Re: invoice 回复：invoice 返回 re