The trouble with "American literature" and "African American literature" as subjects on databases

I've been running hundreds of searches concerning African American literature on the ProQuest Dissertation database over the last week. I noticed that there's a "subject" tab that displays the multiple tags for a topic. For a search for "Toni Morrison" from 2000 - 2018, yields 6,518 dissertation mentions. The "subject" table shows:

American Literature (2,699)
Women’s Studies (1,360)
American Studies (926)
African American Studies (735)
African Americans (652)
[There are dozens more)

I'm curious who decides on the subject areas. I also noticed that "African American literature" is likely embedded within "American literature." That makes sense on the one hand, since African American literature is American literature. However, in English departments, there are in fact often differences and distances. I'm not sure how to account for that in a database though.

Also, if African American literature is embedded within American literature, why is it not embedded in African American Studies? Where do African American literature and African American Studies depart? And where do African American literature and American literature depart as subjects? When is a figure like Toni Morrison a Women's Studies topic and not an African American Studies one?

It would be helpful if database companies offered more information on the reasoning behind subjects and categories.

Related:
• A Notebook on ProQuest Dissertations and Theses