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What’s the difference between a CSA and CSF number?
A CSA number refers to a retiree annuity; a CSF number refers to a survivor annuity. The CSA number is given to federal retirees after they receive their annuity at the time of retirement. An annuitant’s retirement file and accrued benefits are identified by their CSA number, which is assigned by the Office of Personnel Management. All CSA numbers start with CSA, followed by a seven-digit number and suffix of zero. A CSA number is similar to a Social Security number (SSN) and will read as follows: CSA XXXXXXX 0.
When the survivor of the annuity notifies OPM of the spouse’s death and files for annuity, the survivor is given a Retirement Services Reference Card and a CSF number. This number identifies their survivor annuity and starts with CSF, followed by seven digits and the letter W as a suffix. The CSF number reads as follows: CSF XXXXXXX W. For more information on CSA and CSF numbers, call OPM at 1-888-767-6738 between 7:30 a.m. and 5 p.m. ET Monday through Friday, or email OPM at  (from the NARFE national website)