Cataloger's Reference Shelf LCSH - Principles of Structure and Policies for Application
2.0 Functions of Library of Congress Subject Headings
The Library of Congress subject headings system serves both internal and external needs.
2.1 As a controlled vocabulary for subject indexing of the Library of Congress collection
2.2 As a controlled vocabulary for use in subject cataloging and indexing by other libraries or indexing agencies
See also:
Introduction to LCSH