create new dialect determination section

Author: gregsdennis

jsonschema-core.md

@@ -862,7 +862,9 @@ The meta-schema serves two purposes:
 Declaring the vocabularies in use: The `$vocabulary` keyword, when it appears in
 a meta-schema, declares which vocabularies are available to be used in schemas
 that refer to that meta-schema. Vocabularies define keyword semantics, as well
-as their general syntax.
+as their general syntax. By combining various vocabularies, distinct
+sets of keywords can be made available for use in a schema. This collection of
+vocabularies defines a dialect.
 
 Describing valid schema syntax: A schema MUST successfully validate against its
 meta-schema, which constrains the syntax of the available keywords. The syntax
@@ -880,6 +882,30 @@ vocabulary's keywords.
 Meta-schema authoring is an advanced usage of JSON Schema, so the design of
 meta-schema features emphasizes flexibility over simplicity.
 
+### Dialect Determination
+
+When the schema evaluation process begins, the first task is to determine the
+dialect used by the schema. To do this, implementations MUST determine the
+dialect using the following prioritized steps.
+
+1. The `$schema` keyword - Implementations MUST process the schema according to
+   the dialect it declares.
+2. `application/schema+json` media type with a `schema` parameter -
+   Implementations which support media type parameter inputs MUST process the
+   schema according to the dialect the parameter declares. A media type will
+   generally only be available if the schema has been retrieved and only applies
+   to the document root.
+3. Parent dialect - An embedded schema resource which does not itself contain a
+   `$schema` keyword MUST be processed using the same dialect as the schema
+   which contains it.
+4. User configuration - Implementations MAY provide means for the user to
+   configure the dialect under which a schema should be processed.
+
+(Note that steps 2 and 3 are mutually exclusive.)
+
+If the dialect is not specified through one of these methods, the implementation
+MUST refuse to process the schema, as with unsupported required vocabularies.
+
 #### The `$schema` Keyword {#keyword-schema}
 
 The `$schema` keyword is both used as a JSON Schema dialect identifier and as
@@ -900,22 +926,6 @@ The `$schema` keyword SHOULD be used in the document root schema object, and MAY
 be used in the root schema objects of embedded schema resources. When the
 keyword appears in non-resource root schema object, the behavior is undefined.
 
-If present in the resource root schema, an implementation MUST process the
-schema in accordance with the associated dialect.
-
-If absent from the resource root schema, and no parent dialect is defined, the
-schema MUST be processed with the following priorities:
-
-1. If the implementation accepts media type parameter inputs, and an
-   `application/schema+json` media type with a `schema` parameter is available,
-   then the schema MUST be processed in accordance with the dialect indicated by
-   the `schema` parameter.
-2. An implementation MAY offer a mechanism for the user to explicitly set the
-   dialect.
-
-If the dialect is not specified through one of these methods, the implementation
-MUST refuse to process the schema, as with unsupported required vocabularies.
-
 Values for this property are defined elsewhere in this and other documents, and
 by other parties.