The heading that represents precisely the subject content of the work is assigned as the primary subject heading, unless such a heading does not exist and cannot be established. In the absence of a co-extensive heading, a heading broader or more general than the content of the work may be assigned. In this case, the broader or more general heading is the most specific authorized heading in the hierarchy which covers the content of the work. In some cases, several related headings may be assigned. Examples

22.0 Reference

22.1 Specific entry vs. general entry (generic posting)

22.2 Summarization vs. subordinate concept indexing

See also:

A. General Guidelines for Assigning Subject Headings