"...There’s a provision in FERS that allows you to make a deposit, or “buy back”, your active duty military service—and that time will count as creditable service for FERS retirement. Creditable service is important because it’s used to figure out when you can retire with a FERS pension AND how much your monthly pension will be!
Military service with a deposit counts as though you worked under FERS during that time—years, months and days are added to your creditable service. This can really add up to a tidy sum in retirement!..."
Read more at fedsmith.com