The can be a full URL, a reference to a Schema ID defined in an external KML file, or a reference to a Schema ID defined in the same KML file. All of the following specifications are acceptable:
The Schema element is always a child of Document. The ExtendedData element is a child of the Feature that contains the custom data.
This element is used in conjunction with to add typed custom data to a KML Feature. The Schema element (identified by the schemaUrl attribute) declares the custom data type. The actual data objects ("instances" of the custom data) are defined using the SchemaData element.