GEOM label clarification

[PJ]

If I understand correctly from the manual, giving the labels their slice
name (/dev/label/rootfs rather than /dev/ad4s1a) will assure that
regardless of the disk, the boot will be from the disk being booted and
not from another disk as happened to me recently - the fstab on disk ad4
was referncing ad12 so the boot was from ad12 rather than ad4.
The handbook says:
"By permanently labeling the partitions on the boot disk, the system
should be able to continue to boot normally, even if the disk is moved
to another controller or transferred to a different system. For this
example, it is assumed that a single ATA disk is used, which is
currently recognized by the system as ad0."
If the disk is moved to another system, it may no longer be ad0... So
will it still boot correctly?

Or should the ufsid labels be used?

Will both of these contortions work?