Which restriction requires a CDL holder to only drive a vehicle without air brakes?

The restriction that requires a CDL holder to only drive a vehicle without air brakes is represented by the letter "L." This restriction indicates that the individual has not demonstrated the required proficiency or has not passed the necessary tests to operate vehicles equipped with air brake systems. Air brakes are typically found on larger commercial vehicles, and understanding and managing these systems is crucial for safe operation. Therefore, the "L" restriction effectively limits the CDL holder’s driving capabilities to lighter vehicles that do not have air brakes, ensuring both the safety of the driver and other road users until the full skills necessary to operate air brake-equipped vehicles are acquired.

The other restrictions serve different purposes and pertain to other aspects of vehicle operation, which is why they do not apply in this case.