You write a resignation letter when you decide to resign from a job. You send such a resignation letter to your manager or the upper management. It is expected that you mention how grateful you are to the company, what you have learned from it, why you have decided to quit the job, and when you are going to leave. Since this is an official letter, it is also formal in nature.
Title Modified Date
Letter to your manager saying you want to leave - GT Writing Task 1: Sample 51 26 July 2020