Enumeration limits a value space to a specific set of values - if a value isn't specified in the set in the schema, it isn't valid. : enumeration « XML Schema « XML Tutorial

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<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.java2s.com" xmlns="http://www.java2s.com"
  elementFormDefault="qualified">

  <xs:simpleType name="Sizes">
    <xs:restriction base="xs:string">
      <xs:enumeration value="S" />
      <xs:enumeration value="M" />
      <xs:enumeration value="L" />
      <xs:enumeration value="XL" />
    </xs:restriction>
  </xs:simpleType>
</xs:schema>