Unestablished heading:

A heading that is not authorized for use in other MARC records as the lead element of a main, added, series, or subject access field.  An unestablished heading may be a reference to a variant form of the established heading, a form of the heading used only for authority file organizational purposes, or a subject subdivision that is authorized for use with an established heading in an extended subject heading.

In authority records, unestablished headings are used in the 1XX (heading) and 4XX (tracing) fields of reference (008/09, code b or c), subdivision (code d), reference and subdivision (code g), and node label (code e) records.  An unestablished heading may also be used in the 4XX fields of established heading (code a or f) records.