LC call numbers start with a one or two letter code followed by a number and then decimal. The following page has a breakdown of the LC call number schedule for education. The creators of subject heading lists try to categorize all of human knowledge into a logical order. When an item is cataloged in a library collection the cataloger uses this list to determine what subject headings best fit the content of the item. Subject headings apply to the content of any type of communication.
Periodicals can have subject headings as well as books.
Even paintings and music have been given subject headings. There are two major subject organization schemes used in the United States. The Sears Subject Headings and the Library of Congress Subject Headings. Sears list is the older of the two and is used primarily in school and public libraries. Most libraries both academic and public now use the Library of Congress Subject Headings.