A dental service plan corporation may, in the discretion of its board of trustees, through its by-laws, limit or define the classes of persons who shall be eligible to become subscribers, limit and define the benefits which it will furnish, and may divide such benefits as it undertakes to furnish into classes or kinds. In the absence of any such limitation or division of service, such a corporation may provide either or both general and special dental benefits, including such service as may be necessarily incident to dental care. Such corporation may limit the issuance of subscription certificates to residents of any geographic area specified in its by-laws.