Yes, it's often confusing for newbie researchers, because they are put off the "hunt" when they contact NARA in St Louis. For the life of me, I don't understand why they don't include a paragraph in their return letter, informing people that there are other ways of obtaining a veteran's discharge papers, such as contacting their father's (grandfather's, etc.) documents at his local VA, etc. Instead, they make it sound as though it is a dead-end.
NARA is Maryland is a completely separate entity, and as Theron points, they have UNIT records, not individual personnel records.
For instance, I too was turned away by NARA in St Louis, then through careful and persistent research, discovered there were other ways to obtain what I was seeking. I contacted the VA in Michigan to get my dad's discharge papers, and then hired a private researcher to copy several hundred documents regarding the 540th Engineers. It was incredible.