You are completely free to choose where to have your car serviced. Every Cardoen branch has a Cardoen Service Center, where you are always welcome for maintenance and repairs. All you have to do is make an appointment.
You can also have your car serviced at another authorized garage. During the legal warranty, you can go to a brand name dealer or to Cardoen. With an extended warranty, you have to come to a Cardoen branch for maintenance.
If you have a Service+ contract, it is obviously better to come to a Cardoen Service Center, because then the maintenance is free.