Wafer transport by Automated Material Handling Systems is essential in modern semiconductor front-end factories to efficiently supply the manufacturing tools. Most common are ceiling mounted Overhead Hoist Transport Systems that allow high throughput and fast delivery times. These systems are increasingly supplemented by various types of Automated Guided Vehicle Systems that enable high flexibility. This development requires new approaches for transport system dispatching. We denote such Automated Material Handling Systems that combine several types of transport systems as heterogeneous. Here, vehicles with different characteristics can execute either a complete or a part of a transport order. Until now, this task allocation process for the vehicles is executed based on predefined rules. A dynamic exchange between the transport systems based on the current system state is not yet considered. This paper presents a formalization of the related planning problem by a mathematical model for transport system scheduling. The approach is not suitable for real-time decision making. However, on the one hand, it may help to identify and evaluate the solution quality of heuristic approaches. On the other hand, it allows the evaluation of the effect of transfers.