I remember in configuration you assign a specific account group type to driver master, that shows that following account group is specific to driver master, with the same code xd01, which differentiate it from a normal customer.
in that customer master you will found a license category of a driver.
In equipment master data you will assign the license category of the vehicle.
so at the time of shipment if system found two account category it give you a warning or information message.
Look it will help you, if still you have any problem feel free to ask.
regards,