The classification number assigned to a work is determined by the main focus or subject content of the work.
A work dealing with several subjects that fall into different areas of the classification is classed by emphasis, or if emphasis is lacking, by the first subject treated in the work. However, if the work is about a drug, special form of therapy, diagnostic procedure, or environmental condition as a cause of disease, etc., and the disease is only mentioned in the context of the primary subject (the therapy, etc.), classify as always for the primary subject.
A work on a particular disease is classified with the disease, which in turn is classified with the organ or region chiefly affected, regardless of special emphasis on form of therapy or diagnostic procedure used.
Geographic subdivision is provided for certain subjects in the NLM schedules by the application of Geographic Table.
The use of geographical breakdown is restricted to those classes which are annotated with “Geographic Table” in the schedules and includes both monographs and serials.