Using a declaration, you can base your list items on a specific : list « XML Schema « XML Tutorial

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<list itemType="name of simpleType used for validating items in the list">

You specify the type of items by including the itemType attribute. 
itemType attribute should be a reference to a global <simpleType> definition or built-in XML Schema datatype.
The <list> declaration allows you to specify your itemType by creating a local <simpleType> definition.


<simpleType name="ContactTagsType">
  <restriction base="string">
    <enumeration value="author" />
    <enumeration value="xml" />
    <enumeration value="poetry" />
    <enumeration value="consultant" />
    <enumeration value="CGI" />
    <enumeration value="semantics" />
    <enumeration value="employees" />
  </restriction>
</simpleType>



<list> declaration: 

<simpleType name="ContactTagsListType"> 
    <list itemType="contacts:ContactTagsType"/> 
</simpleType>