Two different types of expert may be considered - the genius and the professional. The genius is a very bright person whose skills are largely idiosyncratic. While they may have trained, the types of service they provide is rather particular to that person. Examples of such people are actors, managers, consultants and research staff. The professional, on the other hand, tends to provide a fairly well defined set of skills, albeit quite sophisticated, requiring extensive training.
Examples are doctors, engineers, accountants.
Professionals normally belong to a professional college or other association which has substantial influence in defining and assuring the competence. Thus, organisations employing such people need to recognise that knowledge framework which enable, and also constrains their operations.