Defines a valid class for an interface or Object reference and optionally specifies the type code for the class. If the type code is not specified then the class-id of the class is used (See Mapping an Interface or Object Reference for more information).
<extension key="valid-class" value="Resturant=R" />
<extension key="valid-class" value="Supplier=S" />
In case of the above example, the reference field owner can point to either Restaurant or Supplier, which have been type-coded to "R" and "S" respectively. If the type codes had not been specified then the class-id of the referenced classes would have been used as the referenced id column.