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.
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。
When, for example, a user creates an invoice for a customer without an ID number, most applications reject the entry.
例如，当用户创建一张没有顾客号码的发货单时，多数程序会拒绝输入。
When, for example, a user creates an invoice for a customer without an ID number, most applications reject the entry.
例如，当用户创建一张没有顾客号码的发货单时，多数程序会拒绝输入。
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