Response for a Class
Number of methods in the class plus number of remote methods directly called by methods of the class.
The response set of a class is a set of methods that can potentially be executed in response to a message received by an object of that class. Since RFC specifically includes methods called from outside the class, it is also a measure of the potential communication between the class and other classes.
A large RFC has been found to indicate more faults. Classes with a high RFC are more complex and harder to understand. Testing and debugging is complicated.
A recommended RFC is 55 or less.