<xsd:complexType name="PTA"> <xsd:sequence> <xsd:element ref="PTA.1" minOccurs="0" maxOccurs="1" /> <xsd:element ref="PTA.2" minOccurs="0" maxOccurs="1" /> <xsd:element ref="PTA.3" minOccurs="0" maxOccurs="1" /> </xsd:sequence> </xsd:complexType>