072 Subfield $x - Subject category code subdivision:

Subfield $x contains a code that indicates the level of specificity within the broader category represented by the code in subfield $a.

Example:

008/11   c  [code indicating MeSH]

151  ##  $aLibya

072  ##  $aZ1.$x58.$x266.$x513

072  ##  $aZ1.$x586