Much of the discourse so far is coming from the point of view of what I will call "reactive" photographers. That is, those who go out with a vague or often undefined subject, but react to what they find.
This method of photographing is neither good nor bad it is just a way of working - I often do much of that type of work myself.
However, for many of my projects, I have started with an idea of what I've wanted, sometimes sketched the idea and then put it into a photograph. If you look at Andy Goldworthy and much of John Pfahl's work, it is obvious that they have started with an idea first, that is then executed by finding the subject for the idea. (I am using those two because they could fall into a landscape category as opposed to someone like Joel Witkin.)
How do you classify or quantify a series of photos? I have a friend who shoots a lot of the Hispanic culture, and although he has to find the subject within a set of circumstances, he knows the total presentation he is trying to make within a certain subject genre. For example, he worked for two years on a project documenting prostitutes along the Mexican border. He also has a long term project documenting the Mexican influence upon Native American ceremonies.
In a hierarchy, one would have to say that each of his series started as an idea first that is later realized by finding the subjects to illustrate the idea.
I'm not sure in your method of working any of this makes a difference - or, even why trying to define a hierarchy within a working methodology is important.
Could you explain how/why defining a hierarchy is important to you and your photography? I've never thought about it previously, and it is mildly interesting but what about it is germane to making (better) photos?