In some states, it is an Advance Directive, Health Care Directive or an Advanced Health Care Directive; in others a Medical Power of Attorney, while some have chosen a Durable Medical Power of Attorney. A few states use a variation of Health Care Surrogate. Whatever you call this document, the main purpose remains the same: you name another individual to make health care decisions for you during times when you are unable to participate in the decision.
The person you appoint is in a position of trust and usually involves family members or very close friends. During times when you are incapacitated, for whatever reason, this power kicks in and the person you have designated will then be allowed to confer with doctors, nurses and other medical professionals and make decisions regarding treatment, care providers, placement and other medically related issues. You will want to make sure they know of your preferences and desires and are willing to enforce the same.
BSS can help you determine the proper formalities needed to sign and finalize this document.