劍橋詞典

formal
(especially in business letters ) about; on the subject of
（尤用于商務(wù)信函中）關(guān)于，茲就，事涉
Re your communication of 15 February ... 茲就你2月15日來(lái)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
（用于回復(fù)電子郵件時(shí)的標(biāo)題行，在被回復(fù)的郵件的標(biāo)題之前）
Re: invoice 回復(fù)：invoice 返回 re