Booster Flex is a special Flex cable with a little step-up converter built-in the middle of the cable which doubles the voltage. You should only use it with low current pedals which draw below 50mA, typically Whas and drives. You can get 18V from a single 9V outlet or 24V of an outlet set to 12V. Use the type 1 Booster Flex with the black BOSS standard 5,5/2,1mm centre negative DC-plug with a 9V outlet for the 18V Dunlop Whas, some MXR pedals, Fulltone OCD or others with a low current draw.