The top plug at the rear, which is smaller than the plug on the RH side, is for lubricating the u-joint before you run the engine the first time. I can't recall how much goes in there but ONE CUP rings a bell. The regular fill is on the side.
The transmission fluid normally lubricates the u-joint in use but until it gets a chance to get back to the u-joint it must be pre-lubed via that small hole. You would only do that after a rebuild or fluid drain.
All of the above is well described in the shop manual.
