You can further express occurrence behavior by using the minOccurs and maxOccurs attributes with either the choice element or the element declarations themselves. : choice « XML Schema « XML Tutorial

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2. XML Schema
3. choice

<xsd:element name="middleName">
  <xsd:complexType>
   <xsd:choice maxOccurs="2">
    <xsd:element name="initial" type="xsd:string"/>
    <xsd:element name="name" type="xsd:string"/>
   </xsd:choice>
  </xsd:complexType>
</xsd:element>

The middleName element to contain one of two child elements: initial or name

<middleName>
  <initial>S</initial>
</middleName>
<middleName>
  <name>Stolte</name>
</middleName>

To allow for this content model, we would have to use the choice element:

<xsd:element name="middleName">
  <xsd:complexType>
   <xsd:choice>
    <xsd:element name="initial" type="xsd:string"/>
    <xsd:element name="name" type="xsd:string"/>
   </xsd:choice>
  </xsd:complexType>
</xsd:element>