I'm not quite clear about what you are after, so this may not meet your requirements. There is the Notices system built into Xenforo that you can target at particular usergroups, users, forums, threads or other possibilities. There's also this resource, which does a similar thing but in a slightly different way.
No, you are not clear at all. Your thread title says, "Announce plugin" and you've asked about "warn" and "info". The answers I gave you are correct based on what you have said in your posts.
So rather than getting irritable at me when I am trying to help, I suggest you think about what you want and explain it more clearly. If English isn't your first language, you may find Google Translate useful in helping others to understand what you require.