<xml>
    <RequiredElementA></RequiredElementA>
    <RequiredElementB></RequiredElementB>
    <RequiredElementC></RequiredElementC>
    <OptionalElementD></OptionalElementD>
    <OptionalElementE></OptionalElementE>
</xml>

<xs:complexType name="fixedOrder">
  <xs:sequence>
    <xs:element ref="RequiredElementA"/>
    <xs:element ref="RequiredElementB"/>
    <xs:element ref="RequiredElementC"/>
    <xs:element ref="RequiredElementD" minOccurs="0"/>
    <xs:element ref="RequiredElementE" minOccurs="0"/>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="variableOrder">
  <xs:all>
    <xs:element ref="RequiredElementA"/>
    <xs:element ref="RequiredElementB"/>
    <xs:element ref="RequiredElementC"/>
    <xs:element ref="RequiredElementD" minOccurs="0"/>
    <xs:element ref="RequiredElementE" minOccurs="0"/>
  </xs:all>
</xs:complexType>