optional	If content value is true, then this is a optional item.

idref               	Specifies a reference to the element with the matching <a href=&quot;"idAttribute.

<description>         	Description of the optional item.
<history>             	Specifies the optional history.
<deprecated>          	Specifies that the contents of this element should be marked as deprecated.
<specreference>       	Specification reference for this optional item.
