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@vocab-identifier Points to the Source

For formal vocabularies, the @vocab-identifier attribute value holds a (unique) identifier of the vocabulary, such as (but not limited to) a URI or DOI.

<kwd-group
  vocab="Inspec"
  vocab-identifier=
  "http://www.theiet.org/resources/inspec/about/records/ithesaurus.cfm"
  >
 ?