<Attribute name="attributeName" [tag="*" type=”URL” valuePatterns="*" source=”*”]/>
where
attributeName is the name of the attribute (mandatory)
tag is the name of the tag, where this attribute is present (optional, deafult is * , meaning any tag)
valuePatterns See Using Pattern Matching in Rules.
source is the URI of this XML page (optional, default is *, meaning in any XML page)