The MARC 21 Format for Authority Data identifies three categories of heading usage in records:  main or added entry (008/14); subject added entry (008/15); and series added entry (008/16).  Only established headings may be used as the lead element of access points in bibliographic records.  Name, name/title, and uniform title established headings may be appropriately used as any one, two, or three of the 008/14-16 usage categories.  Topical term and extended subject headings may be used only as subject added entries.   Subdivision headings may be used only in extended subject heading added entries.

See also:

Authority Data Format:  Introduction