You can reference particular fields in a separate form (rather than a complete form) through the use of the <FormRef> element.
Use the <FormRef> element to include another form from within an external form. The following example calls the form named MissingFields.
<FormRef name=’MissingFields’/>
<FieldRef name=’AuthenticationAnswers’/>
<FieldRef name=’AccountInformation’/>
<Field name=’waveset.backgroundSave’>
<Display class=’Hidden’/>
</Field>