20 C.F.R. § 404.1208 – Ineligible employees.

(a) Definition. An ineligible is an employee who, on first occupying a position under a retirement system, is not eligible for membership in that system because of a personal disqualification like age, physical condition, or length of service.
(b) Coverage of ineligible employees. A State may, in its agreement or any modification to the agreement, provide coverage for the services of ineligible employees in one of three ways:
1. (1) As part of or as an addition to an absolute coverage group;
2. (2) As part of a retirement system coverage group covering all positions under the retirement system; or
3. (3) As part of or as an addition to a retirement system coverage group composed of those members in positions in a retirement system who chose coverage.