Social Security-Only means coverage under Social Security without also being covered under either CSRS or FERS. You would have Social Security-Only coverage if you were hired under an appointment that is excluded from CSRS or FERS.
Usually employees serving under temporary appointments (limited to 1 year or less), intermittent employees, and other appointments that would not be expected to last at least 5 years (such as term and excepted indefinite appointments) are excluded from CSRS.
Employees serving under temporary (limited to 1 year or less) appointments and intermittent employees are generally excluded from FERS.
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