• 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).