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The WorldCat Registry supports multiple identifiers for libraries and other cultural institutions, including the international identifier, the ISIL (all prefixes, not just the OCLC prefix), MARC Organization Code, SAN Identifier, NCES Identifier, Australian National Union Catalogue, Australian Business Number, British Library Code and others as they are discovered. As such the Registry is capable of acting as a resolution source for libraries and other cultural institutions and their identifiers.
The WorldCat Registry ID is an identifier in the form of a running number which is assigned to all libraries and other cultural institutions within the WorldCat Registry. It's main purpose is to control records within the Registry and where applicable, their hierarchical linking.
The OCLC symbol is an alpha-numeric identifier with a maximum length of 6 characters. It is assigned to libraries who contribute to WorldCat and is used in WorldCat to identify library holdings. OCLC symbols can be rendered as ISIL (International Standard Identifier for Libraries) by the addition of the prefix "OCLC-" or "O-"for technical encoding in e.g. RFID tags.
To learn more about the other identifiers, please see WorldCat Registry Data Fields Quick Reference