See: http://www.openarchives.org/OAI/openarchivesprotocol.html#Set
The OAI-PMH document states: Repositories may organize items into sets. Set organization may be flat, i.e. a simple list, or hierarchical.
The DRIVER agreement is that hybrid DRIVER repositories that contain metadata-only and metadata-with-full-text resources must support at least one DRIVER set. The DRIVER set is flat and does not have any hierarchical structure. The content of the DRIVER set is Open Access, Freely available resources. Delayed Access resources or Embargoed resources must not be in this list to avoid confusion at the end-user side. The table below shows the preferred setName and setSpec that can be used to create a DRIVER set.

*A harvester only uses the setSpec request to perform selective harvesting. The letters must be in smallcaps.