The Metadata Working Group's schema for hierarchical keywords metadata.

• Exiv2 keys are nested in the form Xmp.mwg-kw.Keywords/mwg-kw:Hierarchy/mwg-kw:<Property>
• The schema namespace URI is http://www.metadataworkinggroup.com/schemas/keywords/
• The preferred schema namespace prefix is mwg-kw

Reference: Guidelines for Handling Image Metadata

Property	Label	Value type	Exiv2 type	Category	Description
Keywords	Keywords	KeywordInfo	XmpText	Internal	Main structure containing keyword based information
Hierarchy	Hierarchy	bag KeywordStruct	XmpBag	External	List of root keyword structures
Keyword	Keyword	Text	XmpText	External	Name of keyword (-node)
Applied	Applied	Boolean	XmpText	External	True if this keyword has been applied, False otherwise. If missing, mwg-kw:Applied is presumed True for leaf nodes and False for ancestor nodes
Children	Children	bag KeywordStruct	XmpBag	External	List of children keyword structures