"nillable" attribute inside of element

[jairton]

Hi,

Could anyone help me?

Given the following example below, What does "nillable" mean?

Thanks,

Jos� airton

<complexType name="Book">
<sequence>
 <element name="isbn" nillable="true" type="xsd:string" /> 
 <element name="year" type="xsd:int" /> 
 <element name="title" nillable="true" type="xsd:string" /> 
</sequence>
</complexType>