## 1 Introduction  
Mixed-model lines are being increasingly used within productive systems due to two related factors: (1) the increasing market demand for a greater variety of products [NO_PRINTED_FORM], and, consequently, (2) the tendency of companies and manufacturers to re-organize their processes according to the ATO (Assembly-To-Order) paradigm. Mixed-model lines allow to produce a large number of variants of a base product with the same assembly system but, at the same time, they increase the pressure on internal logistics processes. In practice, the time and productivity losses generated by missing or defective parts are far greater than for single-model assembly lines, making the correct and on-time delivery of parts to the assembly system even more crucial. Correspondingly, parts feeding systems must become more reactive to volume and mix fluctuations and they must also be able to easily accommodate the introduction of new models and components. Therefore, it is critical to study how different parts feeding policies perform in a mixed-model environment. In particular, this paper, which is an extension of the work of [1] that was presented at the 21 st IFAC World Congress in 2020, will focus on the \"Station-Sequence\" policy. In general, the definition of the parts feeding policy is already a significant challenge within modern assembly systems [2]. Modern parts feeding policies can be grouped into three main categories:  
* The Kanban system, where assembly stations are refilled using the pull Kanban method that is based on bins that contain a fixed quantity of the same item [3]. * The travel kit system, where different parts required for the assembly of the same product are arranged in kits that follow the assembly process of the product through multiple assembly stations. * The stationary kit, where different parts required for the assembly of multiple products are arranged in kits that are directed to a fixed assembly station. One of the variants of the stationary kit category is the so-called \"Station-Sequence\" policy: in this approach, the stationary kits are created with sequences of homogeneous parts (both in terms of typology and physical attributes) that match the sequence of models that are going to be assembled. Figure 1a and 1b show the station-sequence concept and an industrial example, respectively. The station-sequence policy (from now on lowercase and without quotes), is based on the prediction of transportation orders derived from the production sequence. The predicted transportation orders are then used to dynamically generate the tours for the supply of the required parts from the warehouse to the assembly stations. Reference [4] showed that the main differentiating parameter between different parts feeding policies is represented by the total handling time: it includes both the time spent at the warehouse (material handling time at the warehouse), mainly in material preparation, and the time spent at assembly stations (material handling time at assembly stations) for parts picking and placing. With regard to the variation of these two material handling times across the aforementioned categories of parts feeding policies, the literature shows two opposing trends:  
1. The material-handling time at the warehouse increases moving from a Kanban system to a kitting system, due to the increased number of picks that are necessary for the creation of the kits [5; 6]. 2. The material-handling time at assembly stations decreases moving from a Kanban system to a kitting system. In a kitting system, in fact, fewer materials are stored at the assembly station, meaning they can be placed closer to the operator, reducing the fetching time and hence increasing productivity [7]. The station-sequence parts feeding policy is particularly interesting since it retains the low material-handling time at assembly stations typical of kitting systems and combines it with a reduced material-handling time at the warehouse.