a regular exhaust manifold is not high flow. it is made from the factory for a specific amount of air flow that is best suited for the engine from the factory.
You would think that the factory might have considered higher air flow, but the factory decides what is best to put on the car when released. things are considered, like mpg, emissions, etc.
anyway, a header is high flow: ported tubing, bigger tubing bore (some of them), all for higher and faster air flow.
A cat-back exhaust is also made for high flow, due to mandrel bent piping, bigger diameter piping, and high flow muffler. however, ppl think that a cat-back is great all by itself while in reality it's raping your engine unless you have correct intake components to help it out. you will notice a difference if you had no high flow intake with just a cat-back, and vice-versa.
im not sure y its called a turbo-manifold. and im pretty sure you can buy higher flowing turbo-manifolds. and im not sure y they dont call it a turbo-header. but, i was just splitting hairs lol.