The top wireload model is the case where in the nets at the sub system level inherit the top level wire load model even if the sub system has a different wire load model associated with it.
While in the case of enclosed the nets that are completely within the sub system have the wire load model defined for it while the nets that lie both at sub system and top level take that of top level
The segmented is the case where the sub system take the wlm defined for I irrespective of where the nets lie.
Hope this clarifies your doubt biju4u90