Group disability insurance is that form of disability insurance provided by a master policy issued to an employer, to a trustee appointed by an employer or employers, or to an association of employers formed for purposes other than obtaining such insurance, covering, with or without their dependents, the employees, or specified categories of the employees, of such employers or their subsidiaries of affiliates, or issued to a labor union, or to an association of employees formed for purposes other than obtaining such insurance, covering, with or without their dependents, the members, or specified categories of the members, of the labor union or association, or issued pursuant to section 903 of this title. Group disability insurance shall also include such other groups as qualify for group life insurance under the provisions of this title.