"nillable" attribute inside of element.

[jairton]

Given the following example below, what does "nillable" means?

Thanks,

Jose 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>