Section 847.601 - Purpose and scope(a) The purpose of this subpart is to establish the methodology that OPM will use to determine-(1) The cost of an employee's election under § 847.411 or § 847.421 ; and(2) The amount by which annuity payments may be affected as a result of the election.(b) This subpart applies only to CSRS and FERS benefits. The Departments of Defense, and the U.S. Coast Guard will issue regulations providing methodologies for NAFI's under their jurisdictions.