In this article, we consider the growth of microorganisms on mixtures of carbon sources and characterize the consumer-resource relationship for this system. The characteristic features observed for the growth of a single microorganism on a pair of carbon sources allow a representation of this relationship based on a general paradigm for resource classification. This representation is verified using a comprehensive model for microbial growth on carbon sources. The results show that for the same pair of carbon sources the qualitative nature of the consumer-resource relationship changes with changing specific growth rate, and therefore a change in the identity of the rate-limiting substrate(s).