What does 500sl mean in Mercedes 500sl?

The SL-Class is a luxury two-passenger sports car with a convertible top. The SL-Class is offered in two trims; the SL500 and the SL600. The SL500 has a 302 hp 5.0L V8 engine while the SL600 has a 389 hp 6.0L V12 engine. Both trims use a five-speed automatic transmission. The SL-Class has a deployable roll bar behind the seats which reacts the instant it senses the car is in serious danger.