A set of mutually disjoint classes. Disjointness of classes is a special case of disjointness of sets.
(<=> (Class-Partition ?Set-Of-Classes) (And (Set ?Set-Of-Classes) (Forall (?C) (=> (Member ?C ?Set-Of-Classes) (Class ?C))) (Forall (?C1 ?C2) (=> (And (Member ?C1 ?Set-Of-Classes) (Member ?C2 ?Set-Of-Classes) (Not (= ?C1 ?C2))) (Forall (?I) (=> (Instance-Of ?I ?C1) (Not (Instance-Of ?I ?C2))))))))
We want to localize the relationship between sets and classes to just a few axioms, so we use the instance-of machinery here.