A function concept may now be called extensional if and only if it requires functions that are extensionally equivalent at the actual world to be identical. And a function concept may be classified as intensional if and only if it requires intensionally equivalent functions to be identical. Note that these classifications are conceptually different from the distinctions commonly used in the foundations of mathematics. On the terminology used in the foundations of mathematics, functions-as-sets are classified as extensional since they use the axiom of extensionality as their criterion of identity, and functions-as-rules are classified as intensional because they rely on the ostensibly intensional concept of a rule. In the present possible-worlds terminology, function concepts are classified as extensional or intensional based of their behavior at possible-worlds.