Definition: A unification xref defines a reference to an entity in an external resource that has the same biological identity as the referring entity Rationale: Unification xrefs are critically important for data integration. In the future they may be replaced by direct miriam links and rdf:id based identity management. Usage: For example, if one wished to link from a database record, C, describing a chemical compound in a BioPAX data collection to a record, C', describing the same chemical compound in an external database, one would use a unification xref since records C and C' describe the same biological identity. Generally, unification xrefs should be used whenever possible, although there are cases where they might not be useful, such as application to application data exchange.Identity of interactions can be computed based on the identity of its participants. An xref in a protein pointing to a gene, e.g. in the LocusLink database17, would not be a unification xref since the two entities do not have the same biological identity (one is a protein, the other is a gene). Instead, this link should be a captured as a relationship xref. References to an external controlled vocabulary term within the OpenControlledVocabulary class should use a unification xref where possible (e.g. GO:0005737). Examples: An xref in a protein instance pointing to an entry in the Swiss-Prot database, and an xref in an RNA instance pointing to the corresponding RNA sequence in the RefSeq database..
Source:http://www.biopax.org/release/biopax-level3.owl#UnificationXref