Definition: An xref that defines a reference to a publication such as a book, journal article, web page, or software manual. The reference may or may not be in a database, although references to PubMed are preferred when possible. The publication should make a direct reference to the instance it is attached to. Comment: Publication xrefs should make use of PubMed IDs wherever possible. The DB property of an xref to an entry in PubMed should use the string "PubMed" and not "MEDLINE". Examples: PubMed:10234245
Source:http://www.biopax.org/release/biopax-level2.owl#publicationXref