600 Subfield $2 - Source of heading or term:

Subfield $2 contains a MARC code that identifies the source list from which the heading in a 600 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.

Example:

600 17$aNixon, Richard M.,$d1913-1994$2henn