Equivalent URI is a special property in Semantic MediaWiki with a built-in meaning: it marks a page in the wiki as having a well-known meaning beyond this wiki, in an external URI. For example an extension to the Semantic MediaWiki extension might introduce its own property, and all wikis should use the same equivalent URI for it.
If a property in the wiki corresponds to a URI in an existing ontology, you can use SMW's special vocabulary import feature to directly identify it with a property from that ontology. Then RDF Export will simply use that ontology's URI when exporting the property.
Property:Equivalent URI en 1.0