1. A computer-implemented method for building data used by an understanding model of a natural language call routing application, the computer-implemented method comprising: providing a plurality of topic descriptions, wherein each topic description of the plurality of topic descriptions describes a meaning of a topic of a plurality of topics; providing training data, wherein the training data is based on the plurality of topic descriptions; identifying keywords in the training data; and creating an optimal feature set, wherein the optimal feature set is based on the keywords.