The following controls can contain the ref attribute in their XSLT representation:
All other controls MUST NOT contain this attribute in their XSLT representations.
The value of the ref attribute for the multiple-selection list box control MUST be set to a LEAF_XPATH, as specified in section 184.108.40.206.
The value of the ref attribute for the choice group control MUST be set to a GROUP_XPATH, as specified in section 220.127.116.11.
The following W3C XML Schema ([XMLSCHEMA1] section 2.1) fragment specifies the contents of this element.
<xsd:element name="ref" type="xsd:string"/>