re

劍橋詞典

formal
(especially in business letters ) about; on the subject of
（尤用於商務(wù)信函中）關(guān)於，玆就，事涉
Re your communication of 15 February ... 玆就你2月15日來信…

used in the subject line of an email when it is a reply to another email , followed by the subject of the original message
（用於廻複電子郵件時(shí)的標(biāo)題行，在被廻複的郵件的標(biāo)題之前）
Re: invoice 廻複：invoice 返回 re