Creative Commons page recommends/supposes a "verifying" web statement
IPTC/Google Images seem to suppose a web statement of license terms that is not verifying
Creative Commons suggests dc:rights and xmpRights:UsageTerms would have same content. IPTC/Google Images supposes dc:rights contains copyright holder information whereas xmpRights:UsageTerms contains license information.
xmpRights:Marked
xmpRights:WebStatement
xmpRights:UsageTerms
cc:license
cc:morePermissions
cc:attributionURL
cc:attributionName
Some potentially useful links: