Why does a nillable="true" appear in the WSDL?

After generating and deploying a web service to the JBoss appserver
the WSDL shows a nillable="true".

Why?


<xs:element name="aaaRequest" nillable="true" type="tns:aaaRequestType" /> 
<xs:element name="aaaResponse" nillable="true" type="tns:aaaResponseType" /> 
<xs:complexType name="aaaRequestType">
  <xs:sequence>
    <xs:element name="name" type="xs:string" /> 
  </xs:sequence>
</xs:complexType>
<xs:complexType name="aaaResponseType">
  <xs:sequence>
    <xs:element name="return" type="xs:string" /> 
  </xs:sequence>
</xs:complexType>
</xs:schema>
</types>
<message name="TestWs_aaaResponse">
  <part element="tns:aaaResponse" name="aaaResponse" /> 
</message>
<message name="TestWs_aaa">
   <part element="tns:aaaRequest" name="aaaRequest" /> 
</message>

Ben