## 2 Related Work  
Our work is related to the stream of lateral transshipment. [PERSON] et al. [4] give an overview of the literature on lateral transshipments within an echelon and categorize them into reactive and proactive transshipments. Reactive transshipments are a reaction to situations in which a shortage has already occurred. Transshipments are aimed to satisfy demand from other locations. These types of problems are well-observed in the literature [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Proactive transshipments, on the other hand, take place before a demand for parts occurs and should therefore prevent future bottlenecks [8; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34].  
The review [4; 35] identifies various distinguishing features of models and categorizes the existing literature. In addition to the distinction between reactive and proactive transshipment, a distinction is made between the number of echelons, parts, and locations, as well as the cost structure and the backorder option. Another distinguishing feature of transshipment models is the partial or complete pooling in the model. Our model is a proactive transshipment model with one echelon, any number of parts, and two different locations in which no backorders are allowed. It is a complete pooling and a model in which the costs are dependent on the transshipments.  
Transshipment models are applied in various industries, such as the fashion industry [36; 37; 38], retail industry [39; 40; 41], and medical industry [42; 43; 44; 45; 46; 47]. However, the focus of the studies can also be on the transportation mode. While most studies investigate vehicle transport, some authors specialize in container transshipment in shipping [48; 49; 50] or rail transport [51]. The use of transshipments is also analyzed in the special case of logistics service providers [52].  
Our approach is close to the spare parts industry [53; 54; 55; 56; 57], as spare parts, just like prototype parts, are generally only available in small quantities and their absence causes high costs. In both cases, the parts are slow-moving. [PERSON] and [PERSON] [53] find that a proactive transshipment approach is particularly suitable for slow-moving parts.  
The special characteristic of the model of [PERSON] et al. [58] from the automotive industry is the hybrid approach between reactive and proactive transshipments. If reactive transshipments are necessary to satisfy demand, a decision is made on additional proactive transshipments. In this case, proactive transshipments are parts that are transshipped regardless of demand in order to avoid future bottlenecks. [PERSON] et al. [5] have adapted this approach to the logistics of prototype parts. The unique parts, which cannot be stored in different inventory levels at different warehouses at the same time and cannot be reordered, differentiate the model from the existing literature. The focus is therefore on allocating the individual part to the best possible warehouse. The numerical study by [PERSON] et al. [5] showed that transport costs of 5-19% can be saved compared to a strictly reactive approach. However, the use of such an approach faces certain challenges in practice, such as the availability of all data. For this reason, we transfer the approach with assumptions to the industrial use case and then derive decision rules. Certain decision rules have already been established in the literature [59; 60; 61; 62; 63]. However, specific decision rules for unique parts are required for the industrial use case.  
To conclude this section, no existing approach has yet been applied to an industrial use case in prototype parts logistics. Therefore, in the next section, we present the problem setting of prototype part logistics in the automotive industry and adapt the proactive transshipment model [5] to the industrial use case.