I have never in my 15 years of nursing came across a healthy person with a DNR (Im in the UK)
Generally, the DNR is signed by the doctor as part of the patients medical treatment plan. It can apply in hospitals, care facilities and at home/in the community. Facilities may have their own additional forms and steps to take. A DNR is typically for patients who are terminally ill, suffering from an end-stage condition or a persistent vegetative state.
A doctor needs to validate them based on your current medical illness. They usually are reviewed often.
*edit*
DNR = no CPR or mouth to mouth.
Its when someone stops breathing or the heart stops.(most of it is based around the heart stopping) Its usually because the person is too frail to receive cpr and they will probably cause more damage than good even if the person survives.
If its anything else like choking as mentioned above, they will help you. Long as your hearts still beating.
If it was someone bleeding out they would also generally help you. They will help you for anything to prevent you dying, but if your heart stopped or you stopped breathing instantly then they wouldn't be able to help as they aren't allowed to do CPR as the person is already dead and they legally can't do any procedure to bring that person back.