# - Undefined
0 - Library of Congress Subject Headings
1 - LC subject headings for children's literature
2 - Medical Subject Headings
3 - National Agricultural Library subject authority file
4 - Source not specified
5 - Canadian Subject Headings
6 - Répertoire de vedettes-matière
7 - Source specified in subfield $2
|$a - Chronological term (NR)
$v - Form subdivision (R)
$x - General subdivision (R)
$y - Chronological subdivision (R)
$z - Geographic subdivision (R)
|$0 - Record control number (R)
$2 - Source of heading or term (NR)
$3 - Materials specified (NR)
$6 - Linkage (NR)
$8 - Field link and sequence number (R)
Chronological term used as an index term that supplements terms derived from data in the MARC classification record. The term may be referred to in the classification number and caption in field 153 (Classification Number).
May be used to link a classification number with a related chronological heading. The 748 field provides access to classification numbers according to generally accepted thesaurus-building rules (e.g., Library of Congress Subject Headings, (LCSH), Medical Subject Headings (MeSH), Faceted Application of Subject Terminology (FAST)). The values in the second indicator position or in the data in subfield $2 (Source of heading or term) identify the subject heading system or thesaurus used by an institution to formulate the chronological index term. Index terms not controlled by a thesaurus are contained in field 753 (Index Term-Uncontrolled).
Guidelines for applying the content designators and input conventions for field 748 are provided in the MARC 21 Format for Bibliographic Data: field 648 (Subject Added Entry-Chronological Term).