There is no supposed to. If it is a school that you are still very interested in and are on the waitlist or waiting to hear, it could be a good idea to send them an update letter. This lets them know about new things that you have been doing that could help your application and also puts your file on top of a waitlist pile as they put they letter in your folder. However, don't just send a letter to send it. Make sure you have something to say, otherwise you are wasting an ad com's time reading something that will not have any influence on their decision.
No format, but keep it short.
