The ATF tank fitting. Yeah, it is a fancy air vent AND level control of sorts when trying to get the "proper" level of water in a conventional expansion tank. Follow the B&G instructions and learn seven productive methods of pulling out your hair.
Rocky, some systems work just fine without an air eliminator (scoop) fitting, others don't. For the best operation you need several pipe diameters of straight pipe before the air eliminator. Unless you can easily do this I suggest installing the bladder tank without the air eliminator and see how it goes. The best point to connect the bladder tank is before the circulating pump suction.