The subject heading of a letter or complaint refers to THE TOPIC OF THE LETTER.
In letter writing, there are two basic types of letter, formal and informal letters. The informal letter are usually written to loved ones and usually do not require any formality, formal letters on the other hands are usually written to people we are not familiar with or those we did not know at all, thus a high level of formality has to be maintained. Because of this, formal letters have particular format which should be adhered to when one is writing formal letter. A formal letter should have an heading, which is the summarized form of what the letter is about.
