mention that JSON only requires unique IDs per type

doc/guide/Serialization.md

@@ -19,9 +19,11 @@ classDiagram
 ```
 
 The aim of the top level container object is to contain all the referred to objects as well as the general content within
-a single document. It should be noted that this is a different methodology to the way that the 
+a single document. References external to the model would need to be incorporated into the model design with an explicit external reference (hence [this proposal for VO-DML 1.1](https://github.com/ivoa/vo-dml/issues/5) and ). It should be noted that this is a different methodology to the way that the 
 XML was produced in the previous (ant based) versions of this tooling, and as such the XML target namespaces have been changed.
 
+The schema for the serializations can be created with the `gradle vodmlSchema` command ([more detail](./Transformers.md#schema)).
+
 
 ## XML
 
@@ -52,7 +54,7 @@ typically are only unique per table, then the string part of the id is made up f
 
 JSON does not natively have an equivalent to the XML-ID/IDREF mechanism, however, it is possible to distinguish between 
 a named object as the value of a field or a string or integer literal value for the same field which could be interpreted
-as a reference to the object if the data model is known.
+as a reference to the object if the data model is known. An `_id` property is added to referenced types which can then used to make a reference. Unlike the XML case where the ID values need to be unique across the whole document, for the JSON serialization the IDs need only be unique for each type - this makes it somewhat easier to use the native database keys directly.
 
 ```json
 {

doc/guide/Transformers.md

@@ -31,8 +31,10 @@ The gradle task
 ```shell
 gradle vodmlSchema
 ```
-will generate XML and JSON schema for the model. These schema files will automatically be included withing the jar file for the model.
+will generate XML and JSON schema for the model. The schema will be generated in the directory defined by the `outputSchemaDir` property (default `build/generated/sources/vodml/schema/`).
+
+These schema files will automatically be included within the jar file for the model, so that instance validation can be automatically be done without reference to external files.
 
 # Transformation to VO-DML
 
-The transformation of other data model representations to VO-DML is discussed elsewhere. 
+The transformation of other data model representations to VO-DML is [discussed elsewhere](modelling/TransformingToVODML.md).