[arrogantsob]

Not true. There are only two systems and no one can join the older one now. After 1983 there is no more choice everyone is under the SS system.

[kabar]

There is the FERS system and SS plus the TSP. Those are the three legs of the stool. Did you read my post #94?

The FERS replaced CSRS. SS is in addition to FERS. Federal employees pay into both FERS and SS.