If you are in the real-life armed forces of any country, feel free to say so, but only if you are telling the truth. If proof is asked for by an administrator, and no proof is given/found, the user who failed to provide proof would be subject to a ban. Also, if you accuse someone of lying that they are in the real-life armed forces, you must have reason to do so. Any wars which break out over this subject will be dealt with.
Also, if you want to write a bit of fanon about yourself on your userpage, and you state that you are in the armed forces, make sure you make it clear that it is fanon and not the truth.