This means that you can combine model elements of a given type into units that, in turn, you can use as the building blocks to be combined in the same way at the next level of abstraction, and so on.

这就意味着，你可以把一个给定类型的模型元素合并到单元里，依次类推，你可以用这种方式在下个抽象层面上进行合并，并把这些合并后的单元作为一个模块进行使用。