MARC21       650     Subject Added Entry--Topical Term (R)
  Subfields: 
             $a   Topical term or geographic name as entry element
             $b   Topical term following geographic name as entry element
             $v   Form subdivision [NEW]
             $x   General subdivision (R)
             $y   Chronological subdivision (R)
             $z   Geographic subdivision (R)
             $2   Source of heading or term
             $6   Linkage

 Indic/etc.: 1st: Level of subject
             # No information available
             0 No level specified
             1 Primary
             2 Secondary
             2nd: Subject heading system/thesaurus
             0 Library of Congress Subject Headings
             1 LC subject headings for children's literature
             2 Medical Subject Headings
             3 National Agricultural Library subject authority file
             4 Source not specified
             5 Canadian Subject Headings
             6 Repertoire des vedettes-matiere
             7 Source specified in subfield $2