The base type roles are all abstract and should not be used in content.
基本类型角色都是抽象的，并且不应在内容中使用。
Metadata types are those that specific content types aggregate with the base types to provide specific, well-known properties that are specific to a type of content.
元数据类型是特定内容类型与基础类型聚合的类型，提供特定于某类内容的特定属性。
If you need to allow more content structures than the base types allow, you can specialize directly from topic, or form your own base type.
如果您需要比基本类型允许还要多的内容结构，可以直接从topic专门化或形成您自己的基类型。
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