SACO: Subject Authority Cooperative Program of the PPC
SACO was established to allow libraries to contribute proposed subject headings to the Library of Congress.
Of particular interest is: Web Resources for SACO Proposals by Adam L. Schiff.
It is a very rich source of reference materials that you may find useful in developing subject heading proposals or subject classifications for other uses (such as topic maps).
But don’t neglect the materials you find on the SACO homepage.