It is expected that individual communities may formulate well-defined set configurations with perhaps a controlled vocabulary for , and may even develop mechanisms for exposing these to harvesters.
For example, a group of cooperating e-print archives in a specific discipline may agree on sets that arrange metadata in their repositories based on a controlled subject classification.
Each item has an identifier that is unique within the scope of the repository of which it is a constituent. A repository is a network accessible server that can process the 6 OAI-PMH requests in the manner described in this document. A harvester is operated by a service provider as a means of collecting metadata from repositories. To facilitate access to the resource associated with harvested metadata, repositories should use an element in metadata records to establish a linkage between the record (and the identifier of its item) and the identifier (URL, URN, DOI, etc.) of the associated resource. The nature of a resource identifier is outside the scope of the OAI-PMH.