Set-Class is a meta-Class. Its instances are special kinds of classes, all of whose instances are themselves sets (not Classes) such that every member of such a set is specified to be a member of a certain Class.
(<=> (Set-Class ?X0) (Exists (?Thing) (And (Class ?Thing) (Forall (?Things) (<=> (Instance-Of ?Things ?X0) (And (Set ?Things) (Forall (?X) (=> (Member ?X ?Things) (Instance-Of ?X ?Thing)))))))))
(<=> (Eo-Set ?X0) (Exists (?Sc) (And (Set-Class ?Sc) (Instance-Of ?X0 ?Sc))))