Specify whether to show the possible usage of the attribute.
<p>
More exactly, it is the value of the <code>'use'</code> attribute found in the XSD element
of the attribute declaration. For example:
<dl><dd>
<code>
&lt;xs:attribute name="refer" type="xs:QName" <b>use="required"</b>/&gt;
</code>
</dd></dl>

When the <code>'use'</code> attribute is not specified, the default usage is <i>optional</i>.