Name of the thesaurus, ontology, or even the organization that
established a particular group of keywords, for example, “MESH”,
“IEEE”, “author”, “ICD9-codes”, etc.
|Text, numbers, or special characters
source of keywords. This may be an organization (such as
“IEEE”) or the name of a thesaurus, subject index, or ontology
(such as “MESH”).
|Restriction: This attribute may be specified if the element is used.