KERML — isSufficient, semantics, expressiveness

Sufficient types are (Type::isSufficient=true) are described as

A type gives conditions for what things must be in or not in its extent (sufficient and necessary conditions, respectively).

... the type places additional sufficiency conditions on its instances corresponding to all the necessary conditions.

but this semantics is not math/modeled and the syntax cannot specify that only some necessary conditions are to correspond to sufficient ones.