Answer: Reference
The reference notation is optional in a business letter. It is used to draw the reader's attention to a particular point, subject or matter, before the body of the letter.
Reference notations generally take on two forms and they always mean the same thing. They are "Subject: " and "RE:".
The reference notation takes the form "When replying, Refer to" when one needs to apply for a job.
When a person responds to a job advertisement, the reference notation takes the form "In re:". This notation is usually used only in response letters.