regarding the federal retirement systems.
The "offset" part of the retirement system applied to a small number of people. It came into play for employees that had 5 years of federal service, left the government for more than a year, then were rehired after 1983. Those employees had a vested interest in the CSRS but also had to contribute to Social Security. Depending upon age at retirement they got a combination of benefits under CSRS & Social Security. The offset comes into play when Social Security benefits start. The CSRS portion is offset (reduced) by some part of the SS payment.
Federal employees hired before 1984 are covered by CSRS unless they elected to switch to FERS. CSRS employees do not pay nor do they collect SS for the federal service time. They do contribute 7+% of pay into the CSRS system. I believe the contribution is slightly more than SS contributions. They can qualify for SS benefits through other employment outside the federal government.
Federal employees hired as of 1984 are covered by the newer system FERS. It is a combination of SS (same percentage paid by any non-government employee), a small FERS contribution and a Thrift Savings Plan. The TSP is in part an optional % of pay contribution from the employee with a partial matching government contribution.
I Know, I have provide more info than many (any?) of you want. As with most government programs it is very confusing, especially the offset. If Congressmen play be the same rules, you have the basics. I do not know if they have changed any part of this to their benefit. However, I would bet that most would not be covered by the offset portion of the system.