I think there is an error in this paragraph on page (407):

This namespace is often a URL (Uniform/Universal Resource Locator), but it actually can be a URI (Uniform/Universal Resource Identifier). A namespace can be a GUID or a nonsense string such as www-computerzen-com:schema as long as it is unique. Recently, the convention has been to use a URL because URLs are ostensibly unique, thus making the document’s associated schema unique. You’ll find out more about schemas and namespaces in the next section.

The problem is as I see it - actually URL is also a URI.
URI commes in two flavors:

• URN (Uniform Resource Name) – identifies a thing
• URL (Uniform Resource Locator)- provides methods for finding it

So shouldn't it be URN instead of URI ?
Here is a link to wikipedia: http://en.wikipedia.org/wiki/Uniform...rce_identifier

Please let me know if I am correct - because I need to be able to explain the concept for my exam.

Best Regards -- Riceman