X50 Topical Terms - General Information

Field definition and scope:

Each topical term field is unique in its definition and scope. For specific information, select from the list below.

750 Established Heading Linking Entry - Topical Term (R)

Record requirements: The National level record requirement for the topical term fields is Mandatory if applicable, except for field 750 which is Optional. For the national level requirements by subfield, see the specific field.

Field repeatability: See the specific field for its repeatability.

Guidelines for applying content designators:

The content designators identify the subelements occurring in topical term and node label headings constructed according to generally accepted thesaurus-building conventions (e.g., Library of Congress Subject Headings).

Topical terms include occupation and function terms that are used as the lead element of subject or added entry access fields in bibliographic records.

A topical term that is authorized for use only as a subject subdivision part of an extended subject heading is contained in a 180 (General Subdivision) or 182 (Chronological Subdivision) field.

The indicators, subfield codes, and input conventions for the topical term fields 150, 450, 550, and 750 follow similar patterns. Guidelines for applying them are below. Subfields are grouped by type of entry.

For indicator and subfield code definitions and examples, National level (NLR) record requirements, and designator repeatability, see table below. An asterisk ( * ) in the column indicates that the requirement varies. See table in the specific field.:

Position	Description	NLR	Repeatability
Field X50	Topical Terms	*	*
Indicator:
First	Undefined; contains a blank	M

Second
150, 450, 550	Undefined; contains a blank	M

750	Thesaurus	M
0	Library of Congress Subject Headings/Name authority file	A
1	LC subject headings for children's literature	A
2	Medical Subject Headings/NLM name authority file	A
3	National Agricultural Library subject authority file	A
4	Source not specified	A
5	Canadian Subject Headings/LAC name authority file	A
6	Répertoire de vedettes-matière	A
7	Source specified in subfield $2	A

Subfield code:
	Main term portion
$a	Topical term or geographic name entry element	M	NR
$b	Topical term following geographic name entry element	A	NR
$g	Miscellaneous information	A	R

	Subject subdivision portion
$v	Form subdivision	A	R
$x	General subdivision	A	R
$y	Chronological subdivision	A	R
$z	Geographic subdivision	A	R

	Tracing and linking subfields
$i	Relationship information [450, 550, 750]	current - A earlier - A	R
$w	Control subfield [450, 550, 750]	A	NR
$0	Authority record control number or standard number [550, 750]	A	R
$1	Real World Object URI [550, 750]		R
$2	Source of heading or term [750]	A	NR
$4	Relationship [450, 550, 750]	O	R
$5	Institution to which field applies [450, 550, 750]	A	R
$6	Linkage	A	NR
$8	Field link and sequence number	O	R

Input conventions:

Ambiguous headings (what is considered a "topical term")

Punctuation

Spacing

Display constant for X50 fields

Content designator history:

Second Indicator - Nonfiling characters [150, 450, 550] [obsolete]: The second indicator position was made obsolete in fields 150, 450, and 550 in 1993. The values were: 0-9 (Number of nonfiling characters present).

Subfield $g - Miscellaneous information [new, 2014]

Subfield $i - Reference instruction phrase [450, 550] [redefined, 2009]

Subfield $i - Relationship information [750] [new, 2014]

Subfields $u - Record control number [750] [obsolete, 1997] [USMARC only]

$v - Record control number [750] [redefined, 1995] [USMARC only]

$0 - Record control number [550, 750] [new, 1997]

$0 - Record control number [550, 750] [redefined, 2010]

$v - Form subdivision [redefined, 1995]

$3 - Authority record control number [obsolete, 1997] [CAN/MARC only]

In USMARC, from 1993 to 1995, subfield $v was defined as Record control number in the 7XX Heading Linking Entry fields. When subfield $v was adapted for form subdivisions in MARC bibliographic and authority heading fields in 1995, subfield $v was redefined as Form subdivision and subfield $u was defined as Record control number. In 1997, subfield $0 was added as Record control number in 5XX and 7XX fields and $u was made obsolete.

Subfield $1 - Real World Object URI [550, 750] [new, 2017]

Subfield $4 - Relationship code [450, 550] [new, 2009]

Subfield $4 - Relationship code [750] [new, 2014]

Subfield $4 - Relationship code [450, 550, 750] [renamed, 2017] [redescribed, 2017]: Subfield $4 was renamed to Relationship and redescribed to allow for the recording of relationship URIs in addition to MARC and non-MARC codes.