## 3 The Ontology Model for the Generalized Vehicle Routing Problem  
In this section, the ontology system of our generalized model of the Vehicle Routing Problem (VRP) is presented. Some of the ontology systems presented in the literature are limited to specific logistics tasks, for example, monitoring service, city logistics, emergency logistics, manufacturing execution system, and refrigerated goods. Some ontology models try to cover the whole logistics process. As the literature lacks a general ontology model of VRP, the goal of our investigation was to develop a novel general model. We needed a system that would help the user choose which components and metrics of our generalized VRP model to use when transporting a given product, to a given node, with a given vehicle. We also saw the need for the system to help decide which products can be shipped with which vehicles to nodes. The advantage of ontologies is that ontology systems can work together, so our system can be supplemented with a logistics model developed by others. The developed system includes the following key concepts: vehicles, products, period, set of values (static, stochastic, fuzzy, forecasted values), and attributes (nodes, vehicles, period, products, costs, functional parameter) detailed in the mathematical model in our paper [54]. Furthermore, metrics are included in the model. In the sample system, ontological concepts allow for rule-based validation. The main components of the ontology model are shown in Figure 2.  
_The Classes and Their Properties_  
In the following, the classes and their hierarchies and properties are presented. Figures 11 and 12 represent the ontology class hierarchy: Figure 11 shows only the main classes, and Figure 12 also illustrates the subclasses. The figures present how many classes our system contains. These classes will be detailed in the following. Figure 3 illustrates the VOWL representation of our ontology system. The VOWL representation represents ontologies as a graph and provides a tangled representation for large ontologies. The 'BaseComponent' class (Figure 4) presents the base components of the system, which are the following: 'Vehicle', 'Product', 'Node', 'Period', and 'Point'. 'Vehicle' transport products from one node to another. Almost all components also depend on the vehicle, such as the distance between nodes, how long it takes to get from one node to another, and so on. 'Product' is also one of the main building blocks of the system because the products are transported from one node to another. The products affect almost every other component of the system, such as what vehicle to transport or what nodes to visit. 'Period' is a time interval. Within this time interval, the nodes need to be visited several times, because they may have several product demands. 'Period' is also a very important building block because almost every component depends on it. For example, the time it takes to travel between two nodes depends on the period because it does not matter whether the products are delivered during rush hour or at night. 'Point' means that during transport tasks, products are transported from one position to another. Neither class has an object property. The 'Node' class has several subclasses based on out-plant material handling places. The nodes can be classified, such as 'Landfill', 'ResidentialHouse', 'Bank', 'Warehouse', 'Hospital', 'Factory', 'Shop', and 'CustomsPlace'. Some classes may be further specialized, such as 'Warehouse', 'Factory', and 'Shop' classes. The 'Product' class has also several subclasses, depending on what products are shipped. These are the 'People', 'Food', 'Money', 'ElectricProduct', 'BulkProduct', 'Garbage', 'PostalPackage', 'Toys', 'HorticultureProduct', 'Book', 'Furniture', 'Clothing', 'LiveAnimal-Product', 'Medicine'. The 'Vehicle' class also has subclasses according to the transportation units, for example, 'Train', 'WaterCraft', VehicularVehicle', and 'AirCraft'. These classes can be also categorized into subclasses.