DDR Simple API - null aspect name clarification

Regarding Section 3, Vocabularies, it is said

"Property and Aspect names must be unique in their namespace and must conform to the syntax<http://www.w3.org/TR/1999/REC-xml-names-19990114/#NT-NCName> defined in "Namespaces in XML" [XML Namespaces]<http://www.w3.org/TR/2008/WD-DDR-Simple-API-20080404/#ref-Namespaces>. The value __NULL ('_','_','N','U','L','L') is reserved as the null Aspect name."

What is the null aspect name? Under which conditions it should be used?

The document should clarify