I see nothing normative in 4.0, which is intended to be normative I think.  

Proposal: move its text, as amended in other bugs, to end of the intro (where Schema is already being discussed).

Current text:
"SML uses XML Schema [XML Schema Structures, XML Schema Datatypes] to define constraints on the structure of data in a model.

SML scenarios require several features that either do not exist or are not fully supported in XML Schema. These features can be classified as follows:

    *

      References  XML Schema does not have any support for inter-document references, although it does support intra-document references through xs:ID, xs:IDREF, xs:key and xs:keyref. Inter-document references are fundamental to SML since a document is a unit of versioning. SML extends XML Schema to support inter-document references and a set of constraints on inter-document references.
    *

      Rules  XML Schema does not support a language for defining arbitrary rules on the structure and content of XML documents. SML uses Schematron to express assertions on the structure and content of XML documents.

XML Schema supports two forms of extension: "attributes in different namespace" and "application information elements"; both forms are used by SML extensions."