minOccurs in XML schemas

<xs:complexType name="MeshMemberType">
  <xs:sequence>
    <xs:element name="DNSName" type="xs:string" minOccurs="0" />
    <xs:element name="IPAddress" type="xs:string" minOccurs="0" />
    <xs:element name="URI" type="xs:anyURI" minOccurs="0" />
    <xs:element name="IdentifierString" type="xs:string" minOccurs="0" />
  </xs:sequence>
</xs:complexType>