The Content Designator History area of a topic provides a record of significant content designator changes.  The types of changes that are included are:

newly defined content designators that impact on coding consistency within a file, e.g., the location of a meeting entered under a corporate body was not separately subfield coded in the X10 fields prior to the definition of subfield $c in 1980.

redefined codes and values, e.g., in X10 fields, both the number and the name of a part/section of a work were contained in subfield $p prior to the redefinition of subfield $p for only the name of a part/section of a work in 1979.

changes in codes and values for consistency across MARC specifications, e.g., in 008/24-27 for the books specifications, code f (Handbooks) was identified by code h prior to 1979.

changes in repeatability when it impacts on file consistency, e.g., field 020 was not repeatable prior to 1977; multiple ISBN data were contained in repeatable subfields $a, $b, and $c.

restructuring, e.g., field 007 (Physical Description Fixed Field) in 1980.

obsolete content designation, e.g., subfield $b (Number) in the X11 fields when subfield $n was redefined to include meeting numbers in 1979.

Obsolete vs. deleted content designators:

In the MARC formats, a distinction is made between obsolete and deleted content designators.

An obsolete content designator is one that may have been used in MARC records and that may continue to appear in records created prior to the date it was made obsolete.  Obsolete content designators are not used in new records.  An obsolete content designator is recorded in the Content Designator History area.  Content designation instructions are provided for retrospective conversion of records having data elements that would have been identified by the obsolete content designator.

A deleted content designator is not recorded in the history area and will no longer appear anywhere in the MARC documentation.  A content designator that had been reserved in MARC but has not been defined, or one that had been defined but is known with near certainty not to have been used, may be deleted from the format.  A deleted content designator is available for redefinition in a format.

See also:

Bibliographic Data Format:  Introduction