What type of service is creditable as civilian service under FERS?
Generally, any federal (including postal) employment is creditable so long as you were engaged in performing federal functions under authority of an Act of Congress or Executive Order, hired by a federal officer in his or her official capacity as such, and working under the supervision and direction of a federal officer. The general rule is, if a portion of your salary was withheld for federal retirement, that time is creditable as service time.

Time in leave without pay status of up to six months in any calendar year generally is creditable toward retirement. Part-time service time is creditable, prorated according to the time actually worked during that period.

If you had a break in service and withdrew your retirement contributions and later returned, you can recapture the value of that time by making a deposit to the retirement fund, plus interest. The payment may be made in one or more installments. (Note: If you have a CSRS component to your annuity and if the refund was taken before March 1, 1991, you have the option of claiming credit for the time without making this payment, and having your annuity actuarially reduced instead.)

Similarly, you can make a deposit to capture time (for both eligibility and annuity calculation purposes) for which no retirement contributions were taken, but only for time before 1989.

See Chapter 3, Section 3 of the Federal Employees Almanac for information about calculating and making these deposits.