I've always thought that it wasn't possible to have subcategories for only certain categories in a view. It was
all or nothing. I discovered that you can in fact do this. To do this, there is NO subcategory column in the view and you do NOT use a Subcategory field in the document. The trick is to use the "\" character in your Category field. For example, if I had a category called "Section 1" and I wanted to have subcategories "A" and "B", I would assign documents the categories "Section 1\A" and "Section 1\B" The text before the "\" is the category and the text after is the subcategory.