20 CFR §404.1217

1. (a)Absolute coverage group. Generally, the services of an employee covered as a part of an absolute coverage group (see § 404.1205) continue to be covered indefinitely. A position covered as a part of an absolute coverage group continues to be covered even if the position later comes under a retirement system. This includes policemen's and firemen's positions which are covered with an absolute coverage group.
2. (b)Retirement system coverage group. Generally, the services of employees in positions covered as a part of a retirement system coverage group continue to be covered indefinitely. For a retirement system coverage group made up of members who chose coverage, a position continues to be covered until it is removed from the retirement system and is no longer occupied by a member who chose coverage or by a new member of the system. Coverage is not terminated because the positions are later covered under additional retirement systems or removed from coverage under a retirement system, or because the retirement system is abolished with respect to the positions. However, if the retirement system has been abolished, newly created or reclassified positions or positions in a newly created political subdivision cannot be covered as a part of the retirement system coverage group. If the retirement system is not abolished, a newly created or reclassified position is a part of the coverage group if the position would have been a part of the group had it existed earlier. If the retirement system coverage group is made up of members who chose coverage, the newly created or reclassified position is a part of the coverage group if it is occupied by a member who chose coverage or by a new member.