To understand why only the **position light** can be turned on when immobilizing the vehicle on a **hard shoulder outside of town**, it is important to know the proper use of vehicle lights.

- **Position lights**: These lights are mainly used to indicate the presence of the vehicle when it is stopped. They are turned on alone only when the car is stationary, for example, due to a breakdown on a poorly lit road. This helps other drivers see the vehicle and avoid accidents.

- **Conditions of use**: Position lights should not be used alone while the vehicle is in motion. They must always be accompanied by other lights, such as dipped beam or main beam, to ensure adequate visibility.

- **Specific situations**: In the case of immobilizing the vehicle on a hard shoulder outside of town, it is a situation where the car is stopped and there may be little lighting. Here, position lights are sufficient to alert other drivers to the presence of the vehicle.

Therefore, turning on only the position lights is appropriate and safe when the vehicle is immobilized on a hard shoulder outside of town, as it fulfills the function of signaling its presence without the need for other lights.