ALL-VALUES is a way to state all of the the values of a slot on an instance. Its third argument is a set. If all-values are given for a slot on an instance, there cannot be any other values for that slot on that instance. For example, (= (ALL-VALUES i R) (setof v_1 v_2 v_3)) means that R(i,v_1), R(i,v_2), and R(i,v_3) hold, and there is no other unique v_i for which R(i,v_i) holds.
(Nth-Domain All-Values 2 Binary-Relation) (<=> (All-Values ?Instance ?Binary-Relation ?Set-Of-Values) (And (Binary-Relation ?Binary-Relation) (Forall (?Value) (<=> (Member ?Value ?Set-Of-Values) (Holds ?Binary-Relation ?Instance ?Value)))))