610 Subfield $2 - Source of heading or term:

Subfield $2 contains a MARC code that identifies the source list from which the heading in a 610 Subject Added Entry field was assigned. It is used only when the second indicator position contains value 7 (Source specified in subfield $2) . The source of the MARC code is MARC Code List for Relators, Sources, Description Conventions that is maintained by the Library of Congress.