what is this hole for on a B&M Luxos U light?
I now suspect this is a vestigial hole.
I took apart the older version of the same light and found that on that one (which has a 2-conductor cable leading out from the hole), the wires inside are actually soldered onto the back of the main board. I could not determine where the current went along the board from that point, as I was only looking at the back side through a tiny slit.
Clearly they were there, factory installed, for some purpose in prior versions -- maybe a pigtail end for directly wiring the power or tail light. That doesn't seem to make sense since there are spade connectors for those, but maybe that was an option earlier.
I think they are no longer attaching anything to the board inside, and the hole is there because that's how the light lower shell is manufactured, though that still doesn't make a ton of sense to me since it seems like making that hole might involve an extra step after molding. (It sure is clean and precise.)
I'll put a dab of silicone on that hole before I finish the install since there's no point in having extra place for potential water ingress.
EDIT: I took apart one of the newer lights and got a better look at the solder pads to which that terminated wire is attached in a prior light. You can clearly see they've got some kind of connection down around the board to where the main AC / hub power connectors usually attach, so presumably this was a way to wire the light if one chose to solder on a wire or connect up to a factory-soldered lead (I don't know which, but suspect the latter since why would an OEM solder leads then not use them when crimping spade connectors takes 2 minutes?)