Explanatory text and the headings referred to when the linking relationship between
headings from different subject heading systems or thesauri cannot be adequately
conveyed by fields 700-785. No subelements of the heading referred to are
separately subfield coded. Adjacent headings referred to may be contained in a single
subfield $a. The heading referred to may be an established heading or a
subject subdivision that is not equivalent to but is related to the heading in the 1XX
field.
Second - Thesaurus
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