Unique identifier of the controlled vocabulary, taxonomy, ontology, database, thesaurus, index, etc. that is the source of the terms in a group of terms such as a keywords group or subject group. The content is typically (but not limited to) a URI or DOI.
| Value | Meaning |
|---|---|
| Text, numbers, or special characters | A unique identifier of the source of the term named in the element content. This is typically a URI or DOI, but it may be any identifier. |