You write an "informative" or "informing someone" type letter when you want to give some information to someone or give information in response to a request. If the request is from a friend it would be an informal letter, but if you are writing in response to someone you do not personally know, it would be a formal letter.
Title Modified Date
Letter Writing # 196 - Letter to your friend about the museum you have visited 06 August 2021
Letter Writing # 195 - Letter to the friend who has a plan to visit your city 06 August 2021