This upwardcompatibility is useful when you want to combine sets of documentation from two sources, each of which has specialized differently.
如果需要将两种不同来源、分别具有不同专门化的文档组合起来,那么这种向上兼容性非常有用。
3
A concern with evolving XML schema with upwardcompatibility is that XML data cannot be validated with the existence of newly added elements and attributes.