Definition: Used to import terms from external controlled vocabularies (CVs) into the ontology. To support consistency and compatibility, open, freely available CVs should be used whenever possible, such as the Gene Ontology (GO) or other open biological CVs listed on the OBO website (http://obo.sourceforge.net/). Comment: The ID property in unification xrefs to GO and other OBO ontologies should include the ontology name in the ID property (e.g. ID="GO:0005634" instead of ID="0005634").
Source:http://www.biopax.org/release/biopax-level2.owl#openControlledVocabulary