数据类别中的Data Items类似于该基类的子类，比如，一个汽车型号类别可能包含一个表示Ford Focus的数据项以及其他许多数据项。
The data Items within that category are like subclasses of that base; a category for car models will contain a data item which represents a Ford Focus, for instance, as well as many others.
数据类别类似于抽象基类，比如，一个类别可以表示所有的汽车型号。
Data categories are like abstract base classes; a category might be able to represent all models of car, for instance.
材料说明包括材料规格、型号/等级/类别、标称尺寸和适用的材料识别号。
Description of material including material specification, type/grade/class, normal size and material identification applicable.
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