I think part of the problem with any such categorization, is that we often tend to want it to be exclusive... this book is Hard Science Fiction period. In many cases, the book can fit in many categories. Its been a while since I read Downbelow Station, but to me it can fit into a few different categories. It could be, by some measures hard scifi, by other measures, it could be a war story, by others... well you get my point.
To a certain extent, it is often easier to point to books that are clearly not hard science fiction than it is to books that clearly are!