Unfortunately I don't know since I only have the one zone.
However, you'll recall that in my fat clients I have the Enter Event music:zone 1 on each of my music screens. So I think it's the number. That Enter Event of course, just replaces my need to push a zone (1) button.
See if this thread sheds any light:
http://www.xlobby.com/forum/viewtopic.php?t=4637
Failing any help there, I suggest you start a new thread with the X, the error message and the question you just asked (name vs number).