Adding particular vehicle type(s) to a service means that these vehicle types are active for this service and so the customer can choose one of these selected vehicle types when creating a service.
To select desired vehicle types for a service navigate to the admin center:
Admin Center → Services → New record / Actions → Configuration → Vehicle types
For example, for the service of “Box,” two vehicle types are chosen. So in the application, the customers can select one of these vehicle types when creating an order of this type.