Enrollees, and in the case of a retrospective adverse determination, an enrollee's health care provider shall have the right to request an external appeal of a final adverse determination which is made by a health care plan on the grounds that the health care service is not medically necessary or is experimental or investigational. This Subpart shall be applicable to health care plans as defined in subdivision 4-e of section 4900 of the Public Health Law that offer health care services to an enrolled population that is not exclusively covered by title XVIII of the Social Security Act and to external appeal agents certified pursuant to title II of article 49 of the Public Health Law and Insurance Law and this Subpart and to applicants for certification as external appeal agents.