"the dedicated NULL value" ? (ODF all versions)

Dear ODFers,

18.266 mentions the @form:convert-empty-to-null attribute which, when it has a value of "true", indicates "an empty string in the control is regarded as the dedicated NULL value."

What is "the dedicated NULL value"? This term is not defined in ODF and is not a concept from XForms, on which ODF forms appear to be based.

- Alex.


--
Alex Brown
Convenor, ISO/IEC JTC 1 SC 34 WG 1
Editor, ISO/IEC 19757-5 (Extensible Datatypes)