glabel clarification

[PJ]

I understood that labeling a disk with glabel would permit the disk to
be switched to another system and booting from that disk would not
require other manupulations than adjusting network configuration, samba,
rc.conf and a few others..
But what if there is already a disk on the system with the identical
labels in /dev/label/ ?
I understood that whatever the actual disk might be (ad4, ad12,
ad1...)would be irrelevant?
It would appear that the actual booting goes according to the label; so,
if there are duplicate labels the boot will not necessarily be from the
newly installed disk if there is another disk with duplicate glabel labels?
So doing a glabel seems superfluous...
What then is the real purpose of glabel, since the boot process seems to
need a unique identifier?