An attorney-in-fact is a person that has been authorized to make decisions on behalf of another person, which would be legally binding in nature.
The attorney-in-fact is not necessarily a licensed attorney; the main condition is that the attorney-in-fact has the authority of the person that they are representing.
Someone who holds a power of attorney over another person is considered an attorney-in-fact.
Regarding fiduciary responsibilities, the attorney-in-fact makes decisions on behalf of the person who allowed them the right, which will be binding. There is a responsibility on the attorney-in-fact to make decisions only for the benefit of the person that they represent, and to not hinder them through any negative agreement.