655 Subfield $2 - Source of term:

Subfield $2 contains a MARC code that identifies the source list from which the index term 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 Source Codes for Vocabularies, Rules, and Schemes (Genre/Form Code and Term Source Codes) that is maintained by the Library of Congress.

Examples:

655  #7  $aEmblem books$zGermany$y17th century.$2rbgenr

655  #7  $aLithographs$zGermany$y1902.$2gmgpc