It's a unique, 7-digit number that starts with a "CSA" or "CSF" prefix and ends with a number or letter suffix. The combination of these numbers and letters protects annuitants' account security.
Claim numbers may be formatted differently on different OPM documents. They may show partial prefixes (just the "A" or "F"), have spaces or no spaces between characters, or have hyphens or no hyphens (dashes) between characters.