The field types, options, default values, field orders, and so on are defined by the user to attach a custom form of data elements to the document.

字段类型、选项、默认值和字段顺序等都由用户定义，从而为文档添加定制的数据元素。

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Both types of sequences may contain no elements, user-defined types, or they may contain other sequences.

这两种序列都可以不包含元素、用户定义的类型，但可以包含其它序列。

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And it provides registry functions for decorating registered service declarations and elements of the derived content models with user-defined properties, relationships, and classifiers.

而且它还提供注册中心功能，通过用户定义的属性、关系和分类器来扩展已注册的服务声明和派生内容模型元素。

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