XML in a Nutshell, 3rd Edition

Occurrence Constraints

One feature of schemas that should be welcome to DTD developers is the ability to explicitly set the minimum and maximum number of times an element may occur at a particular point in a document using minOccurs and maxOccurs attributes of the xs:element element. For example, this declaration adds an optional middle name to the fullName element:

<xs:element name="fullName">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="first" type="addr:nameComponent"/>
      <xs:element name="middle" type="addr:nameComponent"
          minOccurs="0"/>
      <xs:element name="last" type="addr:nameComponent"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

Notice that the element declaration for the middle element has a minOccurs value of 0. The default value for both minOccurs and maxOccurs is 1, if they are not provided explicitly. Therefore, setting minOccurs to 0 means that the middle element may appear 0 to 1 times. This is equivalent to using the ? operator in a DTD declaration. Another possible value for the maxOccurs attribute is unbounded, which indicates that the element in question may appear an unlimited number of times. This value is used to produce the same effect as the * and + operators in a DTD declaration. The advantage over DTDs comes when you use values other than 0, 1, or unbounded, letting you specify things like “this element must appear at least twice but no more than four times.”