该设计中的数字城市分为四个层次：基础层、元数据层、管理层、应用层。
The design of the Digital City is divided into four levels: basic layer, meta-data layer, the management application layer.
如果需要标识特定类型的类或者增加类的元数据层信息，就必须使用UML扩展。
If you need to identify a special sort of class or add metalevel information to a class, then you must use UML extensions.
为了使一系列给定的软件产品能够有效地参与ISC，并在数据层上进行互操作，就必须在元数据层上能有效地集成。
In order to make a series of given software to participate in ISC effectively, and to interchange on data layer, metadata must be integrated effectively.
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