This one is regarding the "active" careers where you follow your sim. The active career lots (hospital, police station, science facility) are the same throughout your save game. So, for example, I have a sim who is a scientist and has been for a while, he already built the rocket ship etc. I have a new sim who joined that career. He doesn't have to rebuild the rocket because it's still complete from my other sim. So the "new" scientist can just continue with the upgrades on the rocket and enjoy the items my other sim invented that he left at "the office".