extraDocRef
extraDocRef — Allows multiple documents to be associated with a single modification event
Content model
extraDocRef :  refID
     EMPTYAttributes
- 
refID
- The internal identifier of a reference definition element.
 
Description
      A single modification event may have more than one documented
      reference. This element can be used in place of the refID
      attribute in a modificationRecord to record more than one
      refIDs, pointing to the referenced document.