A formal letter is written in a set structure and formal register to communicate with organisations such as government departments, employers, companies, schools, banks or local councils. It is used whenever the matter needs a written record, for example a complaint, request, enquiry, invitation, notification or confirmation.
In Malaysia, formal letters generally follow the layout taught in schools and used by public agencies: the sender's address at the top, a dividing line, the recipient's address, the date, the salutation "Dear Sir/Madam", a bold and underlined subject line, numbered paragraphs, a closing line of thanks and a signature. Because the format is so widely recognised, the recipient can grasp the purpose of the letter at a glance.
A good formal letter uses clear, concise sentences and a courteous tone without unnecessary emotion. State the purpose in the first paragraph, set out the details and any evidence in the paragraphs that follow, and end with the action you are requesting. Include any reference number you have and keep a copy of the letter for your own records.