The zero height USA rail still has a slight offset. The "zero height" means it sits flush to the top of the main rail when installed offset up. Of course you can offset it down as well giving you more options to fit your gear exactly like you want it.
Not all optic mounts will work with USA accessories. If your top mounted optic or accessory has a wide mount or equipped with throw levers or large knobs, it will interfere and you will not be able to place the USA side rail directly next to it. However, the USA rail can fit anywhere along either side of a Razorback for maximum flexibility.
Comes with hardware and wrench as seen in the product photo. Does not include Razorback in installed example photo.