S is Standard / Studio – it will calibrate the first time it is put into a fixture to set a fixed brightness for consistent studio work.
L is Light-Boost / Location – it will output max (about a 35% boost). As the heat builds up in the head it will reduce down to where Standard outputs. With LightBoost you can get far more light output than the size of the fixture normally allows. To maximize the LightBoost, you can dim the fixture down or turn it off between takes.