It is a person you have chosen to make medical treatment decisions for you in the event you cannot do so yourself. This person should agree to carry out your wishes. Anyone who is willing can serve in this capacity for you. It is usually your spouse, an adult child, another family member, or a trusted friend. It should not, and in many states cannot, be your doctor or anyone working in the health care facility.
Some states require it.
Some do not. The Registry feels it shows a more serious intent of the document and should be. Most states require that your two adult witnesses not be related to you, may not be your Health Care Surrogate, and may not be beneficiaries of your estate. Most people give copies of their document to their doctor and family members. The Registry feels registration is as important as signing your Living Will.