524 Subfield $2 - Source of schema used:

Subfield $2 contains a MARC code that identifies the source of the schema used to construct the preferred format or the citation for the item described by the bibliographic record. The source of the MARC code is MARC Code List for Relators, Sources, Description Conventions that is maintained by the Library of Congress.

Examples:

524 ##$aDakota$2usc

524 ##$aC.A.S.$2glc/1988