Re:; reply
Used in email subject lines to indicate a reply; often followed by the original subject.
I changed the subject line to 'Re: About the meeting schedule'.
When replying to an email, 'Re:' is automatically added to the beginning of the subject line.
When you reply, 'Re:' is automatically added to the subject line.
Kana spelling for search convenience; not a practical written form in email subjects.
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.