Currently, OAI-PMH allows selective harvesting only by date.
With sets implementation, it is going to provide an opportunity to harvest by different fields, e.g. location, record type, discovery suppression etc.
Sets configuration should be flexible enough and available via user interface, so that librarians can form their own sets structure. Separate page “Sets” in Settings->OAI-PMH is to be created. Here sets can be searched, viewed, created, duplicated, modified and deleted, according to the user’s permissions (see designs attached).
More information on sets can be found by link http://www.openarchives.org/OAI/2.0/openarchivesprotocol.htm#Set.