So pardon my ignorance but have the leagues been organised in order of last year’s overall rankings?
That is a part of it. The other factor is how involved you are in the site. You need a minimum of 40-50 posts in the previous 12 months to be considered for anything above SCS03. Then a formula kicks in, to combine your previous years Ranking, with your posting.
SCS03 then becomes the highest Ranked members with less than 40-50 posts in the previous 12 months. SCS04 is a combine of both again.