what in the world are "castor sockets"?

[deniss]

Ok, so I didn't have time to fool around with the lower balljoint procedure and decided to pay someone to do it. Turned out that my local MB dealer had the cheapest labor quote of all the shops I've asked ($250 labor per lower balljoint). Now of course they made me buy their own "Mercedes" branded balljoints, which are "Made in France" TRW's with colored clips. Really look the same as Lemfoerders, except the clips. Anyhow, looks like they did the job and said that indeed both the front lower joints had a lot of play. Fine. Then they tell me that I will soon need new "caster sockets"... I think it has something to do with the antiroll/torsion bar up front and the way it's connected to the upper control arm, but I wasn't exactly sure because the service advisor didn't seem to optimistic to explain exactly what that was.

Can anyone tell me what exactly caster sockets are??