Whether the WSDL parser MAY or MUST recognize a foreign EII as a function of (column) whether the namespace of the foreign EII is declared in an @wsdl:required AII within a wsdl:extension EII, and (row) whether the foreign EII itself is annotated with a @wsdl:required AII.

  foreign namespace
(not declared) declared
omitted wsdl:extension @required AII wsdl:extension @required false wsdl:extension @required true
foreign EII omitted @wsdl:required AII MAY MAY MAY MUST
@wsdl:required false MAY MAY MAY MAY
@wsdl:required true MUST MUST MUST MUST

Note that the first column corresponds to WSDL 1.1.