Definition: An xref that defines a reference to an entity in an external resource that does not have the same biological identity as the referring entity. Usage: There is currently no controlled vocabulary of relationship types for BioPAX, although one will be created in the future if a need develops. Examples: A link between a gene G in a BioPAX data collection, and the protein product P of that gene in an external database. This is not a unification xref because G and P are different biological entities (one is a gene and one is a protein). Another example is a relationship xref for a protein that refers to the Gene Ontology biological process, e.g. 'immune response,' that the protein is involved in.
Source:http://www.biopax.org/release/biopax-level3.owl#RelationshipXref