Permission letter purpose is to give or seek permission for some action. Your letter for permission can be rejected if it is not constructed properly.
Permission letter can be written for seeking or giving permission. However, irrespective of its kind the letter needs to be written in formal tone. Letter should clearly mention the purpose of writing the letter.
Letter can be brief and simple and yet persuasive when asking for permission. It is a document that is explicitly necessary for certain actions to take place otherwise it might give rise to various social, economical or legal issues.
It is also important to end the letter with a grateful tone even if the possibility of receiving permission is unlikely.