Classifications of subjects (学科) in the PRC

[anotherslashfan]

As far as I understand, there seems to be a system of classification of (university?/higher education?) subjects, determining 11 different categories of university subjects (大学科门类), and then several "一级学科" (first-level subjects) and many more "二级学科" (second-level subjects). "级" translates as rank, but I don't really understand what this is supposed to mean - what differentiates a "一级学科" from a "二级学科"?
The most info I could find was here: 中华人民共和国学科分类 (Classification of Subjects of the People's Republic of China) but it only relates the facts and doesn't go into the reasons for needing the two ranks.

Any help will be greatly appreciated!