A formal letter writing is a letter written for official purposes in a professional tone like informal or personal letters, formal letters are not flexible. It is a kind of business letter that should be written in a standard format using a conventional language.
Formal letter writing format requires some specific rules and conventions. Also, the language of the letters should be very professional. If you need to write a formal letter, it's important to properly format it and consider who the recipient of your correspondence will be. It's also important to understand when to use this type of letter because it can shape the recipient's perception of you and affect your working relationship with them in the future. A formal letter can be written for various reasons like applying for leave, resignation, job, etc.