DANGER SIGNS: Danger signs indicate that a specific action, task or hazard can lead to serious injury or death.
MANDATORY SIGNS: Mandatory signs indicate that a certain direction must be followed.
CAUTION SIGNS: Caution signs indicate a specific hazard or hazardous situation that is not likely to be life threatening.
PROHIBITION SIGNS: Prohibition signs indicate specific behaviour or actions that are not allowed.
As you can see, the heading is based on the severity of the outcome.