The price of a Product is often dependent upon the procurement of another Product for a particular price, another ProductOffering, or another ProductOffering of the same type.  For example, a Family Plan mobile phone service includes two free phones.  Within this offer, the cell phones are associated with a zero value price.  In another offering, there may be a choice of some number of phone accessories that are offered at no or greatly reduced cost.  <br/>Both of these offers require the employment of a policy rule that governs the price of the accessories.  In the first case, the PolicyRule is used to ensure that two of the phones are provided at zero cost, as long as the terms of a particular ProductOffering are met. In the second case, the PolicyRule defines which accessories can be purchased at what price.<br/>Price rules such as these require another set of business entities in addition to the more basic rules set forth in Product Offering Price and its components.<br/>The intent of this part of the SID model is to present a model that can be used to define simple cross-product price policy rules.  This will be done by extending the SID Policy model. As such, the model presented here is not intended to provide for elaborate pricing schemes.  In these cases, it may be necessary to employ business entities as yet to be defined within the policy model or that may require the development of code specific to the rule.  Still, it is significant that the existing SID Policy model, whose primary use case was to support the definition of networking policies, can be easily extended to meet the needs of a very different domain.<br/>The figure below shows the relationship that a price rule has with ProductOfferings and with Policy.<br/>