|xs:NOTATION, xs:token, xs:Name, xs:NCName, xs:NMTOKEN, xs:ID, xs:IDREF, xs:ENTITY, xs:ENTITIES, xs:QName|
|Defined in: XML Schema part 2|
From the perspective of someone filling out a form, details of XML internals such as NMTOKENs or IDREFs shouldn't matter. Even xs:token (which should have been named xs:tokenized) allows whitespace within the value and doesn't map to anything typical users would enter in a form. Unless you are creating a form that deals directly with XML data structures, choose different datatypes.
Further, xs:NOTATION, xs:ENTITY, and xs:ENTITIES are not guaranteed to be supported at all in XForms, and thus should be studiously avoided.