Matthew
A CV joint is a " constant velocity" joint as found on the front axle of front wheel drive cars.
Tractors usually have double UJ's ( Universal joints) on the front axles because they are far stronger size for size, but even a double UJ does not revolve smoothly during each revolution of the axle when the wheel is not straight ahead. This doesn't matter with a relatively slow turning axle in a tractor, but on a car there would be unacceptable vibration when turning at any speed, hence the fitting of CV joints. Roadless tractors had CV joints in their front axles,because they were sourced from ex-military trucks initially, but they had to have torque limiting slip clutches in the front prop shafts to prevent overloading .
Dave