From the Ministry of Magic

You will recieve and ID number that you should not lose. In the event of emergency, these IDs can save your life. Please keep them in yellow in your signatures. If you attempt to register for conflicting IDs you may be turned down for one, which does not allow you to use that selected feature. For example, you are either and animagus, or another transformative creature. PERIOD. This goes back to the ideas of not having any characters without weaknesses.
Thank you.