http://tools.ietf.org/html/rfc4287#section-6.4

Defines Extension Elements

"Atom allows foreign markup anywhere in an Atom document, except where
it is explicitly forbidden. Child elements of atom:entry, atom:feed,
atom:source, and Person constructs are considered Metadata elements
and are described below. Child elements of Person constructs are
considered to apply to the construct. The role of other foreign
markup is undefined by this specification."