5 C.F.R. § 841.602
Contributions or deductions means the amounts deducted from an employee's pay or deposited as the employee's share of the cost of FERS.
Individual Retirement Record means the record of individual retirement deductions required by § 841.504 .
Last year of service means the calendar year in which deductions stop on the Individual Retirement Record under consideration.
Unexpended balance means the unrefunded amount consisting of-
(a) Retirement deductions made from the basic pay of an employee under subpart E of part 841 of this chapter;
(b) Amounts deposited by an employee for periods of service (including military service) for which-
(1) No retirement deductions were made; or
(2) Deductions were refunded to the employee; and
(c) Interest compounded annually on the deductions and deposits at a rate which, for any calendar year, will be equal to the overall average yield to the Civil Service Retirement Fund during the preceding fiscal year from all obligations purchased by the Secretary of the Treasury during that fiscal year under section 8348(c), (d), and (e) of title 5, United States Code, as determined by the Secretary of the Treasury. Interest on deductions and deposits does not include interest-
(1) If the service covered by the deductions totals 1 year or less; or
(2) For a fractional part of a month in the total service.
Year of the computation means the calendar year when the unexpended balance is being computed.
5 C.F.R. §841.602