The reference line on a business letter is a statement describing the main focus of the letter, a number for an account, invoice, etc. or a reference to previous correspondence. Examples:
RE: Request for refund, order number 12345
Ref: Black Arrow refill number XF8910
REF: Case Number JFL4321
The reference line is optional but can be important for the recipient if an account, invoice, claim, proposal, document number of any kind is the subject of the letter.
The reference line is placed between the recipient address block and the Dear..., leaving blank lines between each.