This is purely speculation on my part, but after reading all the posts about the BB many months ago and no one really knew how full to fill the box, I believe it may be a dip stick of sorts.
Any BB or other differential that I've ever seen are NOT filled to the top as some here have been doing through the upper top orange cap hole.
My guess is that you fill it up until you touch oil with it and that's as full as you fill it.
Again, I'm only guessing as I do not own a M3W and have no direct hands on with one.
Other guesses as to what it is???
