Competency is a legal term, defined as: "duly qualified: having sufficient capacity, ability, or authority [Black's Law Dictionary]. 

Mental capacity is a functional term that may be defined as: the "mental (or cognitive) ability to understand the nature and effects of one's acts."  

There are many ways to assess mental capacity.  Three common models are the philosophical/legal, medical, and functional models. Each has significant benefits and limitations.  PARADISE-2, a behavior-based protocol, maximizes the strengths of previous models, while minimizing their limitations.  Designed for potential litigation settings, PARADISE-2 is used by attorneys, law enforcement, social service personnel, physicians, and nurses in many parts of the Western Hemisphere.  PARADISE-2 was used by the United Nation's International Criminal Tribunal for the former Yugoslavia ("ICTY") in the war-crimes trial of Gen. Pavle Strugar - the first trial of its kind since the Nuremberg Trials.  Due, in part, to PARADISE-2, the Tribunal created new international legal precedent affecting all future war-crime tribunals.