GEOM label clarification

[PJ]

NOW THIS SUCKS.

SUM

# glabel label rootfs/dev/ad12s1a
glabel: Can't store metadata on /dev/ad12s1a: Operation not permitted

This is direct from the manual.... what the "<$#*(@)! is going on?
No identical post on web, but similar say to ignore: "it's harmless?"

I so, why is it there?

There seem to be quite a lot of these kinds of stumbling blocks that are
just plalin annoying...

Is this an annoyance or what for the above situation?