I saw advice on how to restart unity without killing the session.

But I can't find the explanation for the difference between the following:

setsid unity setsid unity --replace setsid compiz --replace

All those commands seem to do the same thing. How are they different?

Is there even any difference in "restarting compiz" vs "restarting unity"?

Thanks.

(I'm using 12.04 if it matters.)