Following are the two Drupal 6 content types that define the USGIN Document Repository (you may want to familiarize yourself with general Drupal concepts):

Collection (ct_collection)

The Collection content type acts as an arbitrary logical unit (or container) that groups the Document-like Information Object (DLIO) nodes which hold the actual data resource. At this time, Collections fulfill 3 objectives at once. Perhaps, they need to be separated at a later point.

Following is a break-down of the Drupal 6 modules used in the USGIN Document Repository site (you may want to familiarize yourself with general Drupal concepts):