650 Subfield $2 - Source of heading or term:

Subfield $2 contains a MARC code that identifies the source list from which the subject added entry 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.

Examples:

650 #7$aEducational buildings$zWashington (D.C.)$y1890-1910.$2lctgm

650 17$aAcoustic measurement.$2test