例如，系统类是不可修改的，而用户定义接口类虽然可以修改，但是不能包含实际的实现代码。
System classes, for instance, are not modifiable, while user-defined interface classes can be modified, but can't include actual implementation code.
数据访问应用程序的其他层不会使用实现类的名称；它们总是引用和使用用户定义接口。
The implementation class name is never used by other layers of the data access application; they always reference and use the user-defined data access interface.
每个转换扩展定义，就像子元素一样，指定实现规则接口的类的规则定义。
Each transformation extension defines, as child elements. a rule definition that specifies a class that implements a rule interface.
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