namespace and multiple schemas import

["learning xml" <learning_xml@hotmail.com>]

Hi, everyone:
I need to use two or more schemas at the same time. So there will be 
different name spaces. As I know, I can use import
<xs: import namespace="http://www.w3.org/XML/1998/namespace";
schemaLocation="myxml.xsd"/>
What is the relationship between schemaLocation and import namespace?
What kind of URI can I choose?  For example, 
xmlns:bk="http://example.org./ns/books"; Is it a .sxd file or something? Can 
I creat such a file in my website and I use it directly?

Thanks in advance!

Wenpeng

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