At first, the logic behind this sounds reasonable: If you change this data element from optional to required, you essentially duplicate error processing that occurs elsewhere in the system.
首先，整个过程背后的逻辑是合理的：如果您将这项数据元素从可选改为必需，那么您基本上就是复制了系统中其他地方所使用的错误处理过程。
These rules describe, for example, which elements are valid, the order of elements within a document, the data types of attributes, whether an element is required or optional, and so on.
这些规则定义哪些元素是有效的、文档中元素的次序、属性的数据类型、一个元素是必需的还是可选的等等。
The <xf:submission> element allows several optional attributes to help control the XML created out of the data model instance by the XForms processor
<xf:submission>元素支持几种可选的属性来帮助控制XForms 处理程序从数据模型实例生成的XML
But it is added as optional so XML data without the cell-phone element is also validated successfully.
但该元素作为可选元素添加，以便没有cell - phone元素的XML数据也可以成功通过验证。
Bad: Sometimes a schema indicates that a data item is optional when some other part of the system downstream fails without the data. Here is an optional element in XML schema.
不好的方法：有时候模式中会指示数据项为可选，但系统中其他的部分却可能因为这部分数据的缺失而失败。
Bad: Sometimes a schema indicates that a data item is optional when some other part of the system downstream fails without the data. Here is an optional element in XML schema.
不好的方法：有时候模式中会指示数据项为可选，但系统中其他的部分却可能因为这部分数据的缺失而失败。
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