Subject headings are specific, predefined terms that the Library of Congress assigns to printed material. The indexers who assign these must use terms that are listed in a controlled vocabulary, and not all indexers will apply the same "tags" to a given book.
Subject headings can be useful in discovering books and materials, but their use can be very limiting. For example, they typically do not include multi-disciplinary or complex topics, and it is impossible to apply a subject heading to every topic that is covered in a given book, so they only identify the overarching subject that a book covers.
Fantasy fiction -- History and criticism
Science fiction -- History and criticism
Climate change
Extinction (Biology)
Renewable energy sources -- Fiction
Environmental protection -- Fiction
Environmentalism in literature
Climatic changes -- In literature
Speculative fiction -- Women authors -- History and criticism
Speculative fiction -- Black authors -- History and criticism
Speculative fiction -- 20th century -- History and criticism
Global warming -- In literature
Environmental literature -- History and criticism