多数用户仅仅关心用什么样的标签和内容，以什么样的顺序，来构造一个有效的UBL文档。
Most users will merely be concerned with what tags and content to put in what order to construct a valid UBL document.
可以为其定义消息序列，表明所需(有效)的操作顺序。
Its message sequence could be defined to indicate the required (valid) ordering of operations.
为了服务的有效使用，服务功能性必须以一种顺序被调用。
There is a sequence in which the service functional capabilities must be invoked for valid use of the service.
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