CSRS creditable service can include:
(for more specific details, see the related articles listed below)

• generally the period of time from each date of appointment to date of separation (when deductions were paid into the retirement system)
• Leave without pay (LWOP) if it does not exceed six months within a calendar year
• Part-time service
• Intermittent service (time actually served, WAE [when-actually-employed]) - up to 260 days a year.
• workers compensation is creditable if the employee returns to duty
• breaks in service of up to 3 days
• Deposit service* (time employee worked for federal government but there were no dedeductions for CSRS retirement).  This is often "temporary" or "casual" work.
• Redeposit service* (employees pays the government back for retirement contributions that were previously withdrawn)
• Military service
• Unused sick leave