A health care proxy (HCP)and a medical power of attorney (MPOA) can be used interchangeably in a document that names a person to make "medical decisions only" for you when you cannot speak.
This person is someone that you trust to follow your medical preferences, regardless if they agree with them. They are your advocate and will speak for you to make sure the medical care you receive, aligns with your wishes in your advance directive.
You do have the power, if you so choose, to have your health care proxy make decisions on your behalf, even if you are not terminally ill or incapacitated.
For the most part, however, a HCP/MPOA normally doesn't have authority to make medical decisions for you until you are incapacitated or unable to communicate.
Trusting your proxy is an important component of this role. Having several conversations to be confident your HCP/MPOA is aware and willing to:
Take the time now, regardless of your health and age, and make sure your advance directive is up to date, and that your health care proxy is aware of all changes in your health, plus has a copy of your advance directive.