Name
extraDocRef — Allows multiple documents to be associated with a single modification event
Content model
extraDocRef :  refID
     EMPTY
Attributes
refID - If an extraDocRef is used, the refID attribute is required.
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.