In RL it stands for Le Mans, signifying a special version for competition in the Le Mans 24hr race. In GT3 it seems to mean what a car WOULD be like IF it were competing at Le Mans. This has been discussed, but if you think that the car wouldn't be an endurance racer (like the Impreza LM), then it's probably like a 'dream car' version of the base car.
Does that help or confuse things further? I gotta go eat!