Definition: The direct source of pathway data or score. Usage: This does not store the trail of sources from the generation of the data to this point, only the last known source, such as a database, tool or algorithm. The xref property may contain a publicationXref referencing a publication describing the data source (e.g. a database publication). A unificationXref may be used when pointing to an entry in a database of databases describing this database. Examples: A database, scoring method or person name.
Source:http://www.biopax.org/release/biopax-level3.owl#Provenance