A vehicle is a set of properties, which together define possible routing on the network. Such as if oneway information should be used, turn restrictions, weight etc.
Multiple "vehicles" can be defined: Car, truck, pedestrian, bicycle etc.
You should choose one of the vehicles to be the active vehicle, the one used during calculations (right-click to choose active vehicle).
In the example below we have a truck, which uses fastest route and acts according to one-way and turn restrictions. Speed profile is the generic, but with an upper limit of 70 km/h.
Weight is 20 x 0.5 = 10 tons.
Height is 35 x 0.1 = 3.5 meters.
Vehicle data is stored inside the configuration file for the network.
If you use a speed field, the presence of the original street network is mandatory. Otherwise the network can be used on its own.
Similar if you use a duration field (in minutes). All values >0 in that field will be used, rather than the original speed for the class.