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://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties. 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 type and value of custom properties are specified by its child variant type XML elements.