is there a default null class for training?

Is there such a thing a default null where a bunch of random real world images are used to make no choice? If we had two label classes class-1 & class-2 our model will always choose one or the other but we need a third class to say it is neither class one or class two. So there should be a default class that cross references & removes any data in this default class that correlates to either of our classes?