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. Booster Flex type 4 with the green 5,5/2,5 centre positive DC-plug is mainy for those 10 older EHX pedals for 24V and should be used with an outlet set to 12V. We suggest you only use Booster Flex if you are short of outlets and don’t have two to make 18V or 24V using Stack Flex or Series adapter Flex.
BE CAREFUL when using Booster Flex. There is a risk of damaging one of your 9V pedals by powering it with too high a voltage. Check the specifications of the pedal and if you still have some questions, you’re welcome to send an e-mail to firstname.lastname@example.org and get some useful information.