What is the extension type system URI?

In Part 1, in section A.2.2 The Element Declaration Component, we can
read:

    A.2.2 The Element Declaration Component

   wsdl.elementDeclaration(element)

   wsdl.elementDeclaration(element,system)

    1. element is the {name} property of the Element Declaration component.

    2. system is the absolute URI of the extension type system used for the
       Element Declaration component. This parameter is absent if XML Schema
       is the type system.

Note that a similar statement is done in section A.2.3 The Type
Definition Component.

Two comments:
- what's the system URI?
- why is it a URI and not an IRI?

I am confused about this system absolute URI. Where is it coming from?
Can we provide an example of a value when XML Schema is not in use?

Let's take our example of use of Relax NG or DTDs in Discussion of
Alternative Schema Languages and Type System Support in WSDL 2.0[1]. I
don't believe that we've defined such a URI.

I understand this to mean that if you use something else then XML
Schema for element or type definition, you need to provide a URI (BTW,
why not an IRI?) for identification with the fragment identifiers.

This needs to be clarified.

  1. http://www.w3.org/TR/2005/NOTE-wsdl20-altschemalangs-20050817/
-- 
Hugo Haas - W3C
mailto:hugo@w3.org - http://www.w3.org/People/Hugo/