Each of the general classes is subdivided as much as necessary. For example, 510-519 is for mathematics. Further subdivisions can be made. The ready reference question for a single piece of information. Usually only one answer to ready reference questions, usually found in reference books.
Research question requires more detailed answer.
Readers' advisory question asks for advice from the librarian rather than just information. This category spans the field of knowledge dealing with the human being as a member of society. Geography and history in the 900's.