in my experience, "offset" is used to make the different inputs play at nearly the same volume level.
Ex.: using my Oppo, the offset is not changed and stays at level 0;
using my sat receiver, i use an offset of +4dB so most TV channels and the music channels play well at Reference level.
when i switch from one source to another, volume level is nearly the same.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery