An instance i MUST-BE-ONE-OF a set of classes S iff i is an instance of at exactly one of the classes. Inside the definition of a class, the form (MUST-BE-ONE-OF ?i (setof C1 C2 ...)) is a convention for stating (exhaustive-subclass-partition C (setof C1 C2 ...)). The two forms are equivalent if each class C1, C2, ... is also defined to be a subclass of C.
(<=> (Must-Be-One-Of ?Instance ?Set-Of-Classes) (And (Can-Be-One-Of ?Instance ?Set-Of-Classes) (Exists (?Class) (And (Member ?Class ?Set-Of-Classes) (Instance-Of ?Instance ?Class)))))