可以添加额外的代码来执行一个检查，检查新值是不是一个用户定义值，用以表明数据库值应该被设置为NULL或针对该列定义的默认值。
Additional code could be added to perform a check to see if the new value is a user-defined value indicating that the database value should be set to NULL or the default value defined for the column.
许多软件为各种选项定义默认值，以便用户可以尽可能快地运行。
Many pieces of software will have default values defined for various options so that the user can get running as quickly as possible.
文档类型和扩展名是用户定义的值，它们应当能够表示新文档类型的意义。
The document type and extension are user-defined values, and they should recommend the new document type.
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