646 Subfield $d - Volumes/dates to which classification practice applies:

Subfield $d contains a statement identifying those items in the series to which the classification practice applies.  Subfield $d is used only when the classification practice contained in subfield $a does not apply to all items of the series.

Example:

130  #0  $aOccasional paper (Chinese University of Hong Kong. Dept. of Geography)

646  ##  $ac$dno. 65-$5DLC

646  ##  $as$dno. 1-64$5DLC

050  #0  $aG1$b.O228$dno. 65-