Default/fixed values for xsi:type and xsi:nil xsi:type must resolve to a type definition xsi:type must resolve to a type definition When an xsi:type attribute appears on an element, and has a QName as its value, but the QName does not resolve to a known type definition, processors are now required to "fall back" to lax validation, using the declared {type definition} of the governing element declaration as the governing type definition. Structures introduces a mechanism for signaling that an element must be accepted as 'valid' when it has no content despite a content type which does not require or even necessarily allow empty content. An element can be 'valid' without content if it has the attribute xsi:nil with the value true. An element so labeled must be empty, but can carry attributes if permitted by the corresponding complex type. default values for xsi:nil