What are the legal requirements for carrying a passenger inside an agricultural tractor cab in Austria?

1. A passenger may be carried on the mudguard if a handhold is fitted
2. Any flat surface inside the cab may be used provided the door is closed
3. The tractor must have an approved passenger seat (Beifahrersitz) entered in the vehicle documents, with a secure handhold and a seat belt where prescribed
4. A passenger seat may be added at any time without any entry in the vehicle documents