XML Pocket Reference, 3rd Edition

Compositors

The xs:sequence element is called a compositor, imposing order on its child xs:element particles. There are two other compositors available: xs:choice and xs:all. The xs:choice element permits one of a list of particles to appear, while xs:all requires all particles to appear but doesn’t put constraints on the order in which they appear. In addition to setting rules for their particles, compositors also act as a group, and you can specify minOccurs or maxOccurs for the group as a whole. (The default value for both minOccurs and maxOccurs is 1.)

If you wanted to define a person element that included both name and nationality but weren’t concerned about the order in which they appeared, you could use:

<xs:element name="person">
    <xs:complexType>
      <xs:all>
        <xs:element ref="name"/>
        <xs:element ref="nationality"/>
      </xs:all>
      <xs:attribute ref="id" use="required"/>
    </xs:complexType>
  </xs:element>

Tip

Notice that the xs:attribute isn’t part of the group. Attributes are part of the type, but the compositors apply only to element content.

If, on the other hand, you wanted to define a person element that could contain your choice of a name or alias, you might use:

<xs:element name="person">
    <xs:complexType>
      <xs:choice minOccurs="0" >
        <xs:element ref="name" />
        <xs:element ref="alias" />
      </xs:choice>
      <xs:attribute ref="id" use="required"/>
    </xs:complexType>
  </xs:element>

The particles inside an xs:sequence or xs:choice may be xs:element, xs:sequence, xs:choice, xs:any, or xs:group elements. ...