I'm back.
Yeah I kinda thought about that later, about the µ symbol. So I guess I was right about it being a micron.

Anyhow, I've been trying some settings out on Antonia for about a half hour now and found out I have to raise the symmetry tolerance in LW to 3.02 µm before I get all polygons on both sides of the face selected by selecting all on only one side. 3.02 microns is still a rather tiny amount and within the range of acceptable symmetry, I'd say. Obviously that's relative to what size LW sees Antonia as being and would differ from app to app. But still, a relative measurement is how it all works anyway, since all 3D apps have a different internal working scale.
That's good, in that now I can at least make morphs using symmetry.

You know, it hadn't even occurred to me to set the symmetry tolerance to something else. Never had to mess with it before, since I usually model in symmetry or mirror one side over. And of course, LW always made it perfectly symmetric, because after all, it was dealing with its own tolerances.

The bad news is that if I make a morph on the right side and load it into Poser and try the Split Morph, while it makes a pair of morph dials, the morph on the left side is all messed up. And vice-versa. I'm assuming that's because Poser's tolerance is far less forgiving. And what others here have said imply that too.
I wouldn't have any idea what Poser's symmetry tolerance is though. Probably would have to ask the developers themselves.

I don't know if this information helps you any. I can try to get a more exact number if you'd like. I mean, symmetry didn't work completely at 3.01 µm, but did at 3.02 µm. It may actually be something like 3.015 µm for all I know. If you need me to start going into the third decimal place to see if I can find a more exact number, let me know.

I haven't tried it in Modo with different settings, but I have every reason to believe the results would be very very close, if not identical. The people who developed Modo are actually most of the original LightWave developers who left Newtek to form Luxology and make Modo. Which is why you'll find a lot of LW users also using Modo, and why the two programs play so well together.