The emergence of the Internet of Things (IoT), Internet of Everything (IoE), and now Tactile Internet (TI) among other new networking paradigms, has been enabled by the architectural changes of the Internet through 5G, 6G and beyond. New network architectures had to be heterogeneous in order to fulfill the tighter requirements for latency, reliability, adaptability, etc, the seamless support for a variety of devices, protocols and applications led to the creation of the multi-layered network architectures that include Non Terrestrial Networks (NTN). Many research questions remain open and routing is one that is especially challenging. One manifestation of this heterogeneity is the diversity of spatio-temporal contacts established for communication between different network entities both inter- and intra-layer. In this paper, we expand the definition of heterogeneous wireless networks. We then propose a novel and rigorous mathematical model for contact classification that captures several aspects of heterogeneity. Finally, we provide real-world application examples and map them to our proposed classification.