A circular letter is a document that's written for a closed group of people. They are often used in business but may also be used in social or political situations.
Circular letters are an older form of written communication. They are used to deliver messages among a group of people such as employees of a company or to management staff. Usually, circular letters will contain important information like company policy changes.
Circular letters may also be used to distribute information to a larger group, such as a community or college campus. They may be posted publicly in order to reach a larger audience. A flyer or leaflet is an example of a circular letter.