RefTypes are attributes that can be used to identify nodes, which are in string representation.
The tree structure of the attribute dx:refType has the following appearance:
<xs:attribute name="refType">
<xs:simpleType>
<xs:restriction>
<xs:simpleType>
<xs:list itemType="xs:string"/>
</xs:simpleType>
<xs:minLength value="1"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>