A display constant is a term, phrase, spacing, or punctuation convention that may be system generated under prescribed circumstances in order to make a visual presentation of data in a record more meaningful to a user.  Display constants are used only for variable data fields.

Suggested display constants and examples are provided in each MARC format.  For example:

in the authority format, certain field tags (e.g., the 4XX and 5XX tracing fields), subfield codes (e.g., the subject subdivision subfields $v, $x, $y, and $z in an extended subject heading), and coded values (e.g., tracings control subfield $w/0, Special relationship) may be used to generate specific display constants.

in the bibliographic format, certain field tags (e.g., field 770, Supplement/Special Issue Entry), indicators (e.g., field 511 Indicator 1, Display constant controller), and subfield codes (e.g., the subject subdivision subfields $x, $y, and $z in subject added entry fields 6XX), may be used to generate specific terms, phrases, and/or spacing or punctuation conventions for the display of a record.

in the classification format, certain field tags, subfield codes (e.g., subfield $c and $z), indicator values (e.g., field 684, Auxiliary Instruction Note, first indicator, Type of note), and coded values (e.g., control subfield $w/0, Special relationship) may be used to generate specific terms, phrases and/or spacing or punctuation conventions for the display of a record.

in the community information format, certain field tags (e.g., field 505), indicators (e.g., field 520 Indicator 1, Display constant controller), and subfield codes (e.g., the subject subdivision subfields $x, $y, and $z in a subject added entry), may be used to generate specific display constants.

Examples of display constants are provided under the heading Input Conventions in the field descriptions.

The use of display constants is determined by each organization or automation system.

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A General Introduction to the MARC Format