Describing XML document structures

Just structures, not types RELAX NG patterns focus on the structure and contents of XML documents, not an abstract type system. This makes it much easier to build schemas which focus squarely on the XML documents, without worrying about the consequences for type systems.
Patterns mix choices, order, options RELAX NG's pattern approach makes it possible to say things like "I want to have either an author child element or an author attribute, but not both."
Interleave RELAX NG brings back the & operator that was lost in the transition from SGML to XML. Because of RELAX NG's mathematical foundations, it's possible to efficiently evaluate statements like "I want a price, a description, and a name, but I don't care in what order they appear." Interleave even supports the mixing of ordered groups.
Namespaces naturally RELAX NG supports namespaces pretty much transparently.
Attributes easily Unlike W3C XML Schema, adding attributes to elements is trivial, requiring no fundamental change in the nature of the declaration.