Question: 
What do you mean when you call Nuggets a classification modeling system
Answer: 

A classification modeling system such as Nuggets creates a model to predict which class one or a set of instances, belongs. An instance is one occurrence of the set of values of the predictor attributes and the prediction is the class to which the predicted attribute for that instance belongs.

It requires that the predicted attribute be "nominal" (also called "categorical"). The predictor attributes, also called independent attributes can be nominal or numeric.