Prior to the application of OHN amendments, the ASN.1 Object Identifier tree (a specific type of RH-name tree) identified each arc by a (mandatory) unambiguous primary integer identifier, and by optional (and possibly ambiguous) human-readable secondary identifiers that were names from the Latin alphabet starting with a lower-case letter. Secondary identifiers could never be used without the associated primary identifier, and were not transferred in computer-to-computer communication. The OHN amendment to X.660 | ISO/IEC 9834-1 extends the identification of an arc to include zero or more "Unicode labels" that are names formed using the full ISO/IEC 10646 character set, and that unambiguously identify the arc among all its siblings. These Unicode labels can be used interchangeably with the primary integer identifiers to identify an object using (as with object identifiers) the identification of the arcs from the root to the object. The resulting name is called an object hierarchical name (OHN), and can be used as an IETF International Resource Identifier (IRI) using the scheme name "ohn:". NOTE - An IRI is a recent IETF concept that is a generalization of the well-known URL/URN. This amendment defines a new ASN.1 type, the IRI-REFERENCE type that carries IRIs in protocol (including but not limited to those defined using OHNs), and can also be used to identify ASN.1 modules or XSD schemas. It also provides the definition of the ohn: scheme for IRIs that is also defined in an RFC in accordance with IETF requirements.