比如，有可能表示InvoiceType (Invoice编号的类型)不允许子类型化，任何人都不能定义新版本的InvoiceType。
For example, it is possible to express that InvoiceType (type of Invoice number) cannot be subtyped, that is, no one can define a new version of InvoiceType.
结构化类型可以包含子类型，以及重用其所有属性并包含自己的特定属性的其他结构化类型。
A structured type can have subtypes, other structured types that reuse all of its attributes and contain their own specific attributes.
正被删除的类型化表不能拥有任何子类型。
The typed table that is being dropped cannot have any subtables.
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