L.6.2.4 Custom Properties

Custom properties allow users to extend pre-defined metadata properties with user-defined properties. Custom properties are stored in the part targeted by the relationship of type: http://purl.oclc.org/ooxml/officeDocument/relationships/customProperties. Each property is represented as a property XML element and uniquely identified through the name, fmtid, and pid attributes. All custom properties are considered complex properties. The datatype and value of custom properties are specified by its child variant type XML elements.