what do "xpath="@xx/"and xpath="./"" mean?

Hi,
I need to add some features such as ref, groups, keyref and unique in XML 
schema, I found a few examples from the Internet. howerver, I don't know 
what do xpath="@xx/"and xpath="./" mean?

In addition, if there are multiple schemas, how can I import them to one 
schema?
what is teh extensioni of complex types?

Thanks,
Wenpeng

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