It is to fill the diff after draining. It is a clear plastic tube with a connector that goes on the end of the diff oil bottle. You can use it to squeeze the new diff oil into the differential. Be sure to check the fit of this "spout" to the differnetial oil bottle.
Where did you get this? I searched for it for years and finally mickey-moused my own filler tube.
When you buy gear oil in the UK, the container comes with about an 8 inch length of clear plastic tube already attached to the container top. I wish I had saved a couple of them when I lived there. Maybe some of you guys in the UK can send us your discarded filler tubes!