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。
Now, look at the XML data that contains an optional element.
现在，看一下包含可选元素的XML数据。
Now, look at the XML data that contains an optional element.
现在，看一下包含可选元素的XML数据。
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