A relation-universe of a relation of any arity is a class from which is drawn every item in every tuple of the relation. Like EXACT-DOMAIN, it is exactly those items and no other. Thus, to state that the universe of a relation is covered by some class, one can state that the relation-universe is a subclass of the covering class.
(<=> (Relation-Universe ?Relation) (And (Relation ?Relation) (Class ?Type-Class) (Forall (?X) (<=> (Exists (?Tuple) (And (Member ?Tuple ?Relation) (Item ?X ?Tuple))) (Instance-Of ?X ?Type-Class)))))
(=> (Relation-Universe $X $Y) (Class $Y)) (=> (Relation-Universe $X $Y) (Relation $X)) (=> (= (Relation-Universe ?Relation) ?Type-Class) (Forall (?X) (<=> (Exists (?Tuple) (And (Member ?Tuple ?Relation) (Item ?X ?Tuple))) (Instance-Of ?X ?Type-Class)))) (=> (= (Relation-Universe ?Relation) ?Type-Class) (Class ?Type-Class)) (=> (= (Relation-Universe ?Relation) ?Type-Class) (Relation ?Relation))