The term manifold is used in many applications. I believe the term came from the early plumbers who helped develop components of the automobile. A manifold is defined as a pipe or chamber having multiple apertures for making connections. Hence, intake manifold or exhaust manifold. You will also find a manifold on a pumper fire truck where several hoses can be attached.
Header also has it's origins in plumbing, were a header is defined as a pipe that serves as a central connection for two or more smaller pipes.
So, is it that a header is made from pipe and a manifold cast the difference in describing these components that do essential the same thing?
To much time on my hands.....