(b) The term "employment" means any service performed by an employee in the employ of the state, any political subdivision thereof, or any instrumentality of either for such employer, except (i) service which in the absence of an agreement entered into under this article would constitute "employment" as defined in the Social Security Act; or (ii) service which under applicable federal law may not be included in an agreement between the state and the Secretary of Health, Education and Welfare entered into under this article; or (iii) service in positions covered by a retirement system established by the state or by a political subdivision or an instrumentality of either on the date the agreement referred to in Section 25-11-7 or any modification of such agreement is made applicable to the coverage group (as defined in Section 218(b)(5) of the Social Security Act) to which the employee performing such services belongs. Service which under the Social Security Act may be included in an agreement only upon certification by the Governor in accordance with Section 218(d)(3) of that act shall be included in the term "employment" if and when the Governor issues, with respect to such service, a certificate to the Secretary of Health, Education and Welfare pursuant to Section 25-11-11(5) of this article. Services, the compensation for which is on a fee basis, may, to the extent permitted by applicable federal law, be excluded in any plan or agreement approved under or authorized by this article.