What does "Re:" in a business letter mean? When should we use it?
Wiktionary lists re as a preposition that means “About, regarding, with reference to; especially in letters and documents”, while OED1 (1914) says:
Re sb² [Ablative of L. res thing, affair.] In the matter of, referring to. The L. phr. in re is similarly used († formerly also = in reality). Re infecta, ‘with the matter unfinished or unaccomplished’, has also been freq. employed in Eng.
Thus, use of re as a preposition meaning about or regarding seems perfectly reasonable to me; it's a concise alternative to regarding, a word which is often used verbosely.
Use re when introducing a new topic. For example, the following would be grammatically and perhaps even stylistically acceptable:
Re item 1, she has no opinion. Re item 2, he doesn't like it. Re item 3, they are sick of it.
Alternative forms of that, with fewer instances of re, might be:
She has no opinion re item 1. He doesn't like item 2. They are sick of item 3.
She has no opinion about item 1. He doesn't like item 2. They are sick of item 3.
RE or Re is just a prefix used before the subject line of a previous email message to mean the new message is a reply for a previous message.
In a business letter, it introduces the subject that it is about.
Re your letter of August 2
RE: Stands for regarding to/or Referring to when used in a business letter (equivalent to Subject line in emails). Was often used in business letters before emails were popular, hence the confusion.
Used as a subject first line of the letter and not in the main text. Example, right before or after salutation:
RE: TAX payments
To whom it may concern,
I have been informed.....(Main text of first paragraph)
Dear MR/MRS X,
RE: Open job position for Arts consultant
I am interested in applying for the above mentioned opening ......(Main text of first paragraph)