For Official Use Only

Sometimes people have information that is very important and they do not want that information to be shared with just anyone. This information could be related to safety, security, or privacy. So when they write down or talk about this information, they mark it as "for official use only," which means that only certain people who have permission can read or hear about it. It's like a secret message that is only meant to be seen by specific people who need to know about it.