subspecialty or subspeciality (British English) is a narrow field of professional knowledge/ skills within a specialty of trade, and is most commonly used to describe the increasingly more diverse medical specialties. A subspecialist is a specialist of a subspecialty.
medicine, subspecialization is particularly common in internal medicine, cardiology, neurology and pathology, and has grown as medical practice has:
become more complex, and
it has become clear that a physician's case volume is negatively associated with their complication rate; that is, complications tend to decrease as the volume of cases per physician goes up.   See also
Notes and references
McHenry CR (2002). "Patient volumes and complications in thyroid surgery". The British Journal of Surgery. 89 (7): 821–3. doi:10.1046/j.1365-2168.2002.02145.x. PMID 12081730. Full Text
Birkmeyer JD; Finlayson EV; Birkmeyer CM (2001). "Volume standards for high-risk surgical procedures: potential benefits of the Leapfrog initiative". Surgery. 130 (3): 415–22. doi:10.1067/msy.2001.117139. PMID 11562662.