The optimiser deals with two kinds of cost: actual (i.e. monetary) costs and hidden costs.
Vehicles, orders and locations can have both actual and hidden costs associated with them.
Hidden costs are considered by the optimiser when making decisions, but they do not contribute to the actual costs reported by the optimiser. Hidden costs are nominally measured in the same units as actual costs.
Hidden costs have many uses, for example they can be used to cause the optimiser to prefer the use of a particular subset of the fleet or used to affect the “shape” of routes.