ENUM in C

I never know ENUM in C can be used in this way.

enum
{
CT3ID = 0,
CT3_HD,
CT3_IM,
CT3_MACHINE,
CT3FORM_ID,
CT3FORM_OUTPUT,
CT3FORM_RETURN,
CT3_REGION,
CT3_POLAR,
CT3_INAIR,
CT3_OUTLINE,
CT3_OPTIMISE,
CT3_COMP,
};

int main(){
printf("%d\n",CT3_IM);
return 0;
}