A testimonial letter is a written recommendation where the author has had personal experience with the person being endorsed. A testimonial letter is used to show that the person is trustworthy and that the services or products they provide are of a high quality.
A testimonial letter is used by a person who is looking for a job, a place in a university, or someone who sells services and products. The letter needs to be written in a formal tone and format. There is a difference between a recommendation letter and a reference letter. A reference letter simply states that a person performs the required duties. A testimonial letter goes deeper and shows that the person does those duties well.