它提供了描述相关资源以及这些资源之间关系的机制。
It provides mechanisms to describe groups of related resources and the relationships between those resources.
此外，还可以用它查找与XML描述器的位置相关的资源。
It can also be used to locate a resource relative to the location of the XML descriptor.
当你的应用程序需要添加内置购买产品时，你或许想要引用这些额外的苹果资源，它们描述了相关的业务，开发指南和需求。
Getting Started with In-App Purchase on iOS and OS X provides an overview of In-App Purchase marketing strategies and business requirements.
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