model root element

I'm a bit puzzled by the structure of the <model> element.  My primary
question is, must this model, and the subsequent instance document, be a
valid XML document, or simply a fragment?

In essence, what I am asking is, must the <model> element only have one
child, which would be the documentElement of the resulting document?  Or, it
is legal for <model> to have multiple children, thus making the resulting
instance a fragment, and not a valid document itself?

I would hope that this would be illegal:

<model>
	<group name="purchaseOrder">
	...
	</group>
	<group name="contactInfo">
	...
	</group>
</model>

- Joe