Within variable data fields, two kinds of content designators are used to interpret, supplement, or delineate the data found in the field.  They are indicator positions and subfield codes.  Indicators and subfield codes are not used in variable control fields.

Types of content designators:

Indicator positions are the first two character positions at the beginning of each variable data field.  Indicators contain values which interpret or supplement the data found in the field.

Indicator values are interpreted independently, that is, meaning is not ascribed to the two indicators taken together.  For example, in the bibliographic format, tag 245 (Title), if the indicators were 10, the first indicator value of 1 means that a title added entry would be generated, and the second indicator value of 0 means there are no nonfiling characters at the beginning of the title.

Indicator values may be a lowercase alphabetic or a numeric character.  A blank (ASCII SPACE) is used in an undefined indicator position.  In a defined indicator position, a blank may be assigned a meaning, or may mean No information provided.  A blank is represented by the pound sign ( # ) in the Cataloger's Reference Shelf.

Subfield codes are two characters that precede each data element within a field that require separate manipulation.  A subfield code consists of a delimiter (ASCII 1F hex), represented in the Cataloger's Reference Shelf by the dollar sign ( $ ), followed by a data element identifier.  The data element identifier may be a lowercase alphabetic or a numeric character.

Subfield codes are defined independently for each field; however, parallel meanings are preserved whenever possible.  For example, in the bibliographic format, subfield $a is named and defined as Personal name for all X00 Personal Name fields (100, 400, 600, 700, 800).

Subfield codes are defined for purposes of identification of data in the field, not arrangement of the data.  Thus, the order of subfield codes is not necessarily alphabetical, but is generally specified by standards for the data content:  the cataloging rules (bibliographic and authority); the classification scheme (classification); subject heading system/thesaurus rules (community information); and Z39.71 (holdings).

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A General Introduction to the MARC Format