Modularity

Multiple top-level elements. Vocabularies which allow instances that may be rooted at more than one element.

Schema modularity. Schemas that can be composed of distinct schema modules.

Content model extensibility. The ability for one schema module to extend or modify the content of an element declared in another module.

Subtype/equivalence relationships. The ability to express that two elements are in some sense the same kind of object.

Context-dependency. Content-dependent content models (local element declarations).