Power of attorney is the authority given to someone of the patient's choosing to be responsible for his / her affairs in the event of illness or frailty of mind or body rendering them unable to manage those affairs themselves.
It is legally binding and should preferably be drawn up before it is needed with the assistance of a lawyer and the full knowledge of the person given the authority.
Lawyers may sometimes advise that the person nominated to look after affairs should preferably not be a beneficiary in the patient's will.
Randall F, Downie RS. Palliative care ethics: a companion for all specialties. Oxford, London and New York. Oxford University Press 1999. Pages 191-198.