Subunit:

A part of a collection, especially an archival unit described collectively elsewhere in the system.  A subunit may be items, folders, boxes, archival series, subgroups, or subcollections.  The record for a subunit contains fields that describe the subunit and data that identify the host item.

Code d in Leader/07 indicates that the bibliographic level of the record is subunit.

See also:

773  Host Item Entry