What does the letter 'X' preceding the hazard identification number (e.g. 'X338') on an orange plate indicate?

1. The substance reacts dangerously with water, so water must not be used on a fire involving this cargo
2. The substance is water-soluble, so a spill can safely be diluted with water
3. The load is an explosive requiring a police escort
4. The plate is a spare blank plate and the vehicle is currently running empty