Clarify how JSON Schema works with a superset of the defined data model

Author: awwright

jsonschema-core.xml

@@ -200,6 +200,15 @@
                 <t>
                     A JSON document to which a schema is applied is known as an "instance".
                 </t>
+                <t>
+                    JSON Schema is defined over "application/json" or compatible documents,
+                    including media types with the "+json" structured syntax suffix.
+                </t>
+                <t>
+                    Among these, this specification defines the "application/schema-instance+json"
+                    media type which defines handling for fragments in the URI,
+                    and the "schema" media type parameter.
+                </t>
 
                 <section title="Instance Data Model">
                     <t>
@@ -243,25 +252,6 @@
                     </t>
                 </section>
 
-                <section title="Instance Media Types">
-                    <t>
-                        JSON Schema is designed to fully work with "application/json" documents,
-                        as well as media types using the "+json" structured syntax suffix.
-                    </t>
-                    <t>
-                        Some functionality that is useful for working with schemas is
-                        defined by each media type, namely media type parameters and
-                        URI fragment identifier syntax and semantics.  These features are
-                        useful in content negotiation and in calculating URIs for specific
-                        locations within an instance, respectively.
-                    </t>
-                    <t>
-                        This specification defines the "application/schema-instance+json"
-                        media type in order to allow instance authors to take full advantage
-                        of parameters and fragment identifiers for these purposes.
-                    </t>
-                </section>
-
                 <section title="Instance Equality">
                     <t>
                         Two JSON instances are said to be equal if and only if they are of the same type
@@ -288,6 +278,37 @@
                         zeros) are insignificant.
                     </t>
                 </section>
+
+                <section title="Non-JSON Instances">
+                    <t>
+                        It is possible to use JSON Schema with a superset of the JSON Schema data model,
+                        where an instance may be outside any of the six JSON data types.
+                    </t>
+                    <t>
+                        In this case, annotations still apply; but validation keywords will not be useful.
+                        Validations that apply only to one of the core types will always pass,
+                        and other keywords will fail.
+                    </t>
+                    <t>
+                        A custom vocabulary may define support for a superset of the core data model.
+                        The schema itself may only be expressible in this superset;
+                        for example, to make use of the "const" keyword.
+                    </t>
+                    <t>
+                        New data model types may be defined as either supersets of an existing type
+                        (similar to how "number" is a superset of "integer"),
+                        or a type disjoint from the core data model altogether (like "binary").
+                        Existing keywords may be updated to define behavior for the new range of values,
+                        but vocabularies that intend to be compatible with JSON Schema Validation should not
+                        change the behavior for existing types.
+                    </t>
+                    <t>
+                        Drawing on the "binary" example,
+                        a custom vocabulary may define what it means to use a "binary" data type
+                        in the "const" keyword, or the meaning of "minLength" when the instance is a "binary",
+                        but for the sake of consistency, it should not change the behavior when the instance is a string.
+                    </t>
+                </section>
             </section>
 
             <section title="JSON Schema Documents" anchor="schema-document">