XML Schema 1.1 : Conditional Type Assignment revisited

One of the other features of XML Schema 1.1, that I like very much is "conditional type assignment", or CTA. The only requirement is, that there must be an attribute on an XML element to use this feature.

Here is a very simple example.

I'm directly writing an XML Schema 1.1 document below using CTA:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

   <xs:element name="X">
        <xs:alternative type="Type1" test="@xa = 1"/>
        <xs:alternative type="Type2" test="@xa = 2"/>
        <xs:alternative type="xs:error"/>
   </xs:element>
  
   <xs:complexType name="Type1">
        <xs:sequence>
            <xs:element name="a" type="xs:int"/>
            <xs:element name="b" type="xs:int"/>
        </xs:sequence>
        <xs:attribute name="xa" type="xs:int"/>
   </xs:complexType>
  
   <xs:complexType name="Type2">
        <xs:sequence>
            <xs:element name="p" type="xs:int"/>
            <xs:element name="q" type="xs:int"/>
        </xs:sequence>
        <xs:attribute name="xa" type="xs:int"/>
   </xs:complexType>
 
</xs:schema>

The requirement of XML Schema 1.1 validation in this case is: If the attribute "xa" on element "X" has value 1, then element "X" has a certain type. If the value of attribute "xa" is 2, then element "X" has another type.

The two valid XML documents for the given XML Schema document are following:

<?xml version="1.0" encoding="UTF-8"?>
<X xa="1">
    <a>1</a>
    <b>2</b> 
</X>

and,

<?xml version="1.0" encoding="UTF-8"?>
<X xa="2">
  <p>1</p>
  <q>2</q> 
</X>

For anything else as value of attribute "xa", or infact any other kind of content the element "X" will be assigned the type xs:error (which makes the element "X" invalid").