Multi-model systems may arise in a number of configurations. Other configurations are also possible. In these cases, however, a persistent problem that exists is one of heterogeneous models (and simulators based on them) having to exchange information with each other, but that differences exist in how the individual models represent information about the systems they are describing. A chief distinguishing feature is simply the difference between the representation of time and process flow - the difference between discrete time models and continuous time models. Setting aside that difference, there are other dimensions of heterogeneity that may be enumerated. These are presented, along with indicators of some current examples/approaches to mitigate the differences.