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Notes: Dublin Core: Recommended best practice is to identify the resource by means of a string conforming to a formal identification system. Audubon Core: An arbitrary code that is unique for the resource, with the resource being either a provider, collection, or media item. Using multiple identifiers implies that they have a same-as relationship, i.e. they all identify the same object (e. g. an object may have an http-URL, an lsid-URI, and a UUID-number).
Required for media collections, No for media resources (but preferred if available).
Constraints in the context of Dublin Core: