Incorporate overiew of schema structure into specification

3 Guide to the Schema Structures
The EMIX information exchange model defines common structures that can be used to define products whose value varies with the time of delivery. Because the future of smart energy markets its not known, there is an emphasis on extensibility and composition to allow EMIX to be suitable for markets known and unknown, and for easy evolution.
The EMIX 1.0 Specification consists of three schemas. The EMIX schema defines the framework and extensibility as well as agreement types common to many markets. The Power schema defines the specific information exchanges, based on the EMIX framework, needed for markets in power and energy. The Resource schema defines how load and generation, specific capabilities of devices and systems to affect power and energy markets, can be described irrespective of the underlying technologies.
Other markets, particularly other products for energy markets, share the characteristic that value is closely linked to time of delivery. Power and Resource provide examples of extension and conformance with the EMIX model. Specifications that wish to claim conformance with EMIX use should follow the same approaches. Information exchanges based on specifications that conform to the EMIX specification, can be used within any business process or specification that uses or exchanges EMIX payloads.
3.1 Core Extension Elements
The core extension elements are the Product Description and the emixBase. These types are described using the abstract types Item (itemBase), and the Interface (emixInterface). Almost all of EMIX is assembled from these four abstract types.
The abstract Product Description type is the basis for all static descriptions of EMIX products. Product Descriptions are static in that they refer to a particular instance in time. Most of the elements in the Power and Resource schemas are creating Product Descriptions for Power Markets.
The abstract emixBase defines how to apply a Product Descriptions to a Schedule. That schedule may be as simple as a single 5 minute interval on a particular day, or as complex and repeating you can find in your own personal calendar. Any type derived from the emixBase can hold any Product Description. Information elements derived from the EmixBase include Products, Options, TEMIX, and Delivery (Meter Information).
The ItemBase is the basis for the lowest level description of each Product and its aspects. The term Item is in common business use for that thing on a line of a purchase order, or of a receipt, or on a bill of lading. ItemBase derived types have at least a name, a unit of measure, and a scale factor. The Power schema defines 3 power types and three energy types derived from ItemBase.
All product descriptions include the EmixInterface. The EmixInterface is where something transfers ownership for the market. In Power, it can be a node or meter, and aggregation of nodes or meters, a pair of nodes, or a geographic area. Other specifications can derive from the base type to support their own needs. Any type derived from the Interface can be used in any of the EmixBase derived types.