Extensions are not a required feature of the grammar and, thus, their syntax is not as well defined as the other constructs of the language.
扩展并不是文法必需的特性，因此它们的语法不象该语言的其它构造那样得到良好定义。
2
While you are developing your schema grammar, you should set this feature to true so that Xerces2 will check all the constraints necessary to determine if your schema is valid.