noun
Re:; reply
Used in email subject lines to indicate a reply; often followed by the original subject.
件名を「Ｒｅ：会議の日程について」に変更しました。
I changed the subject line to 'Re: About the meeting schedule'.
メールの返信では、自動的に「Ｒｅ：」が件名の先頭に付きます。
When replying to an email, 'Re:' is automatically added to the beginning of the subject line.
ASCII variant commonly seen in email clients; same reading and usage.
From English 'Re:', an abbreviation of 'regarding' or 'in reference to', adopted into Japanese email conventions.