From what I can see its a separate accreditation. So you can be LEED GA + Sites AP VS LEED ND. I might be wrong though.
That's what it seems like to me too.
SITES is specific to the site (from the building skin out) and can apply to projects that would not be certifiable in LEED such as parks, streetscape and similar. Go to sustainable sites to find out more. Especially review the pilot certified projects.
I have LEED ND, and thought that process was much more difficult. When I took it I had to purchase a course that gave me experience in a LEED project. SITES just required the exam, which was not difficult if you have a history of working with or studying sustainability principles. I would say LEED ND is more allied to planning and community, whereas SITES is more allied to landscape and environment.