Hierarchical or nested keyword, in which several levels of a taxonomy that pertains to the standard can be described.
Language in Keywords: Keywords are contained in <kwd-group> elements. There may be several <kwd-group>s, each of which can be identified separately by language, vocabulary, or taxonomy source. Since different languages could mean, for example, different writing directions, none of the individual keyword elements (<kwd>, <compound-kwd>, <nested-kwd>) take the @xml:lang attribute; that is reserved for the <kwd-group> element. This means that if there are keywords in several languages they should be in different <kwd-group>s.
Vocabulary Attributes: For controlled vocabularies, two attributes can be used to link a keyword to its term source:
- @vocab-term — This attribute holds the canonical form of the keyword as expressed in the vocabulary named in the <kwd-group>. The content of the keyword element (<kwd>, <compound-kwd>, <nested-kwd>) might not be exactly the same as the canonical form, as the keyword element content might be a user-specific variant, for example, the term in another language.
- @vocab-term-identifier — This attribute holds a unique identifier and possible pointer to the specific term in the named vocabulary (typically a URI or DOI reference), but @vocab-term-identifier could be an item number or other system-specific identifier.
... <kwd-group kwd-group-type="xyw-food-taxonomy"> <nested-kwd> <kwd>beverages</kwd> <nested-kwd> <kwd>alcoholic</kwd> <nested-kwd> <kwd>beer</kwd> <kwd>martini</kwd> <kwd>wine</kwd> </nested-kwd> </nested-kwd> <nested-kwd> <kwd>dairy</kwd> <nested-kwd> <kwd>milk</kwd> <kwd>drinkable yogurt</kwd> <kwd>milkshakes</kwd> </nested-kwd> </nested-kwd> </nested-kwd> </kwd-group> ...